tag:theconversation.com,2011:/africa/topics/twins-3193/articlesTwins – The Conversation2024-02-04T19:09:23Ztag:theconversation.com,2011:article/2222792024-02-04T19:09:23Z2024-02-04T19:09:23ZShould twins be in separate classes? Many schools say yes, but the answer is not so simple<figure><img src="https://images.theconversation.com/files/572101/original/file-20240130-27-kgghse.jpg?ixlib=rb-1.1.0&rect=51%2C22%2C4870%2C3254&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.pexels.com/photo/close-up-shot-of-two-girls-having-an-online-class-while-writing-9037329/">Alena Darmel/Pexels</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><blockquote>
<p>Should my twins be in the same class at school? </p>
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<p>As a clinical psychologist specialising in twins, this is one of the most frequent questions parents ask me.</p>
<p>Many schools continue to separate twins due to a deep-seated belief it is better for the development of <a href="https://pubmed.ncbi.nlm.nih.gov/16697537/">separate identities</a>. Both <a href="https://www.tandfonline.com/doi/pdf/10.1080/00094056.2010.10523142">research evidence</a> and clinical experience tells us it is not so simple.</p>
<h2>How many twins are there?</h2>
<p>What happens to twins is not a niche issue. In Australia, twins represent approximately <a href="https://raisingchildren.net.au/pregnancy/health-wellbeing/twin-pregnancy/twins">one in every 80</a> pregnancies. According to the <a href="https://www.abs.gov.au/statistics/people/population/births-australia/latest-release">Australian Bureau of Statistics</a>, 1.4% (4,286) of pregnancies were multiple births in 2022, with the vast majority of these being twins. </p>
<p>As these statistics suggest, each year there will be many parents who have to navigate what happens to their kids at school and many teachers will have a twin in their classroom.</p>
<h2>Old school rules for twins</h2>
<p>Traditionally, schools did not tend to ask parents for their views when <a href="https://journals.sagepub.com/doi/epub/10.1177/0895904813510778">placing twins in classes</a>.</p>
<p>This approach was based on anecdotal experience, misguided perceptions and beliefs, and/or <a href="https://www.tandfonline.com/doi/pdf/10.1080/00094056.2010.10523142">limited research</a> suggesting that <a href="https://journals.sagepub.com/doi/epub/10.1177/0895904813510778">being apart was better</a> for twins’ development and academic performance. </p>
<p>Still today, some parents tell me school principals insist on placing twins in separate classes because they believe it is better for shaping their individual identities. There is also the often unspoken rationale (particularly for identical twins) that it is easier for teachers and students to tell them apart.</p>
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Read more:
<a href="https://theconversation.com/curious-kids-why-are-some-twins-identical-and-some-not-121435">Curious Kids: why are some twins identical and some not?</a>
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<h2>What does the research say?</h2>
<p>When looking at the research about twins at school, the findings tell us a different story. There is <a href="https://www.jstor.org/stable/20721129">little evidence</a> to suggest twins perform better academically when they are in separate classes. The <a href="https://www.sciencedirect.com/science/article/pii/S037837820600096X?via%3Dihub">exception might be</a> when one twin has special needs or when there is an unhealthy amount of competition between the twins. </p>
<p>A <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9493409/pdf/10.1177_08959048211015626.pdf">Canadian study</a> published in 2022 found teaching primary school-aged twins in the same classroom had some positive impact on their behaviour and how they relate to others. This makes sense when we consider many twins have had limited experience being away from each other before starting school. So they are likely to feel more secure if placed together in these early transition years. </p>
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Read more:
<a href="https://theconversation.com/whether-its-a-new-teacher-or-class-heres-what-to-do-when-your-child-is-not-loving-it-199288">Whether it's a new teacher or class – here's what to do when your child is not loving it</a>
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<h2>What parents, schools should be doing instead</h2>
<p>In 2022, the Australian Multiple Birth Association (a non-profit organisation) released a <a href="https://www.amba.org.au/position-statements/class-placement">policy statement</a>, noting: </p>
<ul>
<li><p>there is no one-sized fits all answer</p></li>
<li><p>parents “are best placed” to determine what will suit their children</p></li>
<li><p>schools should consult parents each year about where their children should go.</p></li>
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<p>Twins may also have a view, particularly as they get older. Therefore, listening to each twin will be an important part of the decision-making process. Although, what one twin says they want might not be what they really want or need (depending on the nature of the twin dynamic). For example, the twin who says they want to be in a separate class to their co-twin might actually be the twin who wants to stay together. Such is the enigma of the twin relationship!</p>
<p>This makes it even more important to gather as much informed information as possible before making a decision. For schools, the message is no fixed policy is best when it comes to welcoming twins into your school.</p><img src="https://counter.theconversation.com/content/222279/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Catherine E. Wood has presented at the Australian Multiple Birth Association national conference at different times.
</span></em></p>As a clinical psychologist specialising in twins, this is is one of the most frequent questions I am asked.Catherine E. Wood, Associate Professor and Clinical Psychologist, Swinburne University of TechnologyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2210122024-01-15T23:33:57Z2024-01-15T23:33:57ZNetflix’s You Are What You Eat uses a twin study. Here’s why studying twins is so important for science<figure><img src="https://images.theconversation.com/files/569230/original/file-20240115-23-hyyxnq.png?ixlib=rb-1.1.0&rect=753%2C126%2C2891%2C1891&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Still from You Are What You Eat.</span> <span class="attribution"><span class="source">Netflix</span></span></figcaption></figure><p>A new Netflix documentary, You Are What You Eat, showcases sets of identical twins as they adopt different diets. For eight weeks one twin follows a vegan diet while the other one follows an omnivorous diet. The experiment is compelling because, being genetically identical, the health of each twin is very similar before the trial. </p>
<p>I won’t spoil the ending for those who haven’t seen it, but if you prefer the drab writings of academics over the glitz and glamour of Netflix, you can read the published paper <a href="https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2812392">in the journal JAMA Network Open</a>. </p>
<p>The documentary underscores the extraordinary contribution twin studies make to advancing our understanding of the world. But this goes well beyond comparing different diets. </p>
<p>British polymath Sir Francis Galton <a href="https://academic.oup.com/ije/article/41/4/905/686858?login=false">first documented</a> the striking similarities of twins in 1875, arguing this proved “nature” was an important contributor to our dispositions and health. Since then, twins have been used extensively in research. What is it then that makes twins so special, and how do researchers harness the power of twins? </p>
<h2>Twins as comparisons for each other</h2>
<p>The Netflix documentary highlights one important feature of twins – they are good for making comparisons. Identical twins share nearly all of their genes, and usually grow up in the same household, meaning they experience the same parenting, schooling and so on.</p>
<p>The documentary is an example of a <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6235704/#:%7E:text=Randomized%20controlled%20trials%20(RCT)%20are,between%20an%20intervention%20and%20outcome.">randomised controlled trial</a>, where participants are selected at random for some intervention (like a new drug) and those not selected serve as controls (in drug trials they might get a placebo). Randomised trials are normally seen as the gold standard in evaluating what works.</p>
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Read more:
<a href="https://theconversation.com/randomised-control-trials-what-makes-them-the-gold-standard-in-medical-research-78913">Randomised control trials: what makes them the gold standard in medical research?</a>
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<p>We don’t actually need identical twins for such trials. In fact, <a href="https://www.sciencedirect.com/science/article/pii/B9780128215142000039">relatively few</a> trials use twins. But twins can help to ensure the treatment and control groups are as similar as possible. This is especially important when there are few participants. In the Netflix study, there were only 44. Without twins, their results would have been more uncertain. </p>
<p>For ethical reasons or just sheer practicality, we can’t always run randomised trials. This is where twins can help us out. Say we want to know the effect of <a href="https://www.jstor.org/stable/2118189">education on earnings</a>, or the effect of <a href="https://pubmed.ncbi.nlm.nih.gov/29054888/">smoking on developing lung cancer</a>.</p>
<p>These questions, and many more, have been tackled by comparing the differences within sets of identical twins (for example, where one twin smokes and the other doesn’t). </p>
<p>By focusing on differences between twins, we eliminate genetic and common family factors, and can be more confident about causality. Using twins doesn’t fully solve the problem of omitted factors, but it helps to reduce it. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/569236/original/file-20240115-15-8y5sv3.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Two women with blonde dreadlocks in a commercial kitchen with various food trays in front of them" src="https://images.theconversation.com/files/569236/original/file-20240115-15-8y5sv3.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/569236/original/file-20240115-15-8y5sv3.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=317&fit=crop&dpr=1 600w, https://images.theconversation.com/files/569236/original/file-20240115-15-8y5sv3.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=317&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/569236/original/file-20240115-15-8y5sv3.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=317&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/569236/original/file-20240115-15-8y5sv3.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=398&fit=crop&dpr=1 754w, https://images.theconversation.com/files/569236/original/file-20240115-15-8y5sv3.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=398&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/569236/original/file-20240115-15-8y5sv3.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=398&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Still image from You Are What You Eat.</span>
<span class="attribution"><span class="source">Netflix</span></span>
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</figure>
<h2>Twins and heritability</h2>
<p>Galton’s fascination with twins stemmed from wanting to know why we are the way we are. Is it our genes (nature), or is it our upbringing and personal experiences (nurture)? His early observations were informal, but eventually researchers developed sophisticated methods to disentangle genes and environment. </p>
<p>The main approach is essentially to see whether identical twins, who are genetically the same, are more similar than fraternal twins, who on average share only 50% of the gene variants that make us unique. If the identical twins are more similar, that indicates genes matter.</p>
<p>For example, twin studies have shown that around <a href="https://psycnet.apa.org/record/2015-20360-001">40% of individual differences in personality</a> can be explained by genes, along with as much as <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3275599">80–90% of differences in height and weight</a> at the end of childhood.</p>
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Read more:
<a href="https://theconversation.com/curious-kids-why-do-people-grow-to-certain-sizes-105131">Curious Kids: Why do people grow to certain sizes?</a>
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</em>
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<h2>Limitations and misuse of twin research</h2>
<p>One drawback to twin studies is that twins are a select group, and findings may not always generalise to the broader population. Only about <a href="https://www.aihw.gov.au/reports/mothers-babies/australias-mothers-babies/contents/focus-population-groups/mothers-who-have-multiple-births">1.4% of births in Australia are twins</a>. </p>
<p>Twin heritability studies also rely on <a href="https://www.apa.org/monitor/apr04/second">strong assumptions</a>, like that the common family environment matters to the same extent for identical and fraternal twins. This can be overcome by focusing on <a href="https://www.science.org/doi/10.1126/science.2218526?url_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org&rfr_dat=cr_pub%20%200pubmed">identical twins reared apart</a>, but these studies are rare.</p>
<p>There has also been some misunderstanding and misuse of findings from twin studies. Galton’s belief in the power of nature led him to promote <a href="https://en.wikipedia.org/wiki/Eugenics">eugenics</a>, the idea of selective breeding to achieve “genetic superiority”, which had <a href="https://encyclopedia.ushmm.org/content/en/article/eugenics">devastating consequences</a> in the 20th century. </p>
<p>But our genes are not our destiny. While a certain combination of genes may raise the likelihood you’re extraverted, who we grow up to be is a complicated interplay between genes, upbringing and personal experiences. Even disorders with a genetic basis, like Alzheimer’s, are rarely certain to occur, and we can take actions to reduce our risk.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/569237/original/file-20240115-19-i7cn48.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A person carrying two baby carriers with identical twin babies" src="https://images.theconversation.com/files/569237/original/file-20240115-19-i7cn48.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/569237/original/file-20240115-19-i7cn48.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/569237/original/file-20240115-19-i7cn48.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/569237/original/file-20240115-19-i7cn48.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/569237/original/file-20240115-19-i7cn48.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=501&fit=crop&dpr=1 754w, https://images.theconversation.com/files/569237/original/file-20240115-19-i7cn48.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=501&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/569237/original/file-20240115-19-i7cn48.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=501&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Identical twins are actually quite rare.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/father-carrying-twin-babies-car-seats-1128705452">Shutterstock</a></span>
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<h2>The future of twin research</h2>
<p>Twin studies have been conducted for decades, with <a href="https://www.sciencedirect.com/science/article/pii/B978012821514200009X">more than 60 twin registries</a> operating around the world. In Australia, the largest registry is <a href="https://twins.org.au/">Twins Research Australia</a>, who have around 75,000 members. </p>
<p>With more opportunities to link data, genome mapping, and advances in machine learning, what we can learn from twins will only increase in the future. And we can expect twins to continue to play a key role in advancing our understanding of the world for many years to come.</p><img src="https://counter.theconversation.com/content/221012/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Nathan Kettlewell does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>The new Netflix documentary follows identical twins as they adopt different diets. This is a great example of a twin study – a uniquely useful research tool in science.Nathan Kettlewell, Senior lecturer, Economics Department, University of Technology SydneyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2061822023-11-09T01:26:42Z2023-11-09T01:26:42ZIt’s hard to find a surrogate in Australia. But heading overseas comes with risks<figure><img src="https://images.theconversation.com/files/532120/original/file-20230615-27-n0wdqe.jpg?ixlib=rb-1.1.0&rect=21%2C133%2C4656%2C3331&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://unsplash.com/photos/FqqaJI9OxMI">Unsplash/Kelly Sikkema</a></span></figcaption></figure><p>Surrogacy offers the chance of parenthood for those who can’t carry a pregnancy for medical or social reasons. </p>
<p>In a surrogacy arrangement, a surrogate gives birth to a baby for the intended parents to raise. Most intended parents are heterosexual or gay couples, but single people can also use surrogacy to have a child. </p>
<p>In the 2021–2022 financial year, <a href="https://www.homeaffairs.gov.au/foi/files/2022/fa-220800210-document-released.PDF">213</a> Australian babies were born through international surrogacy – an arrangement between Australian intended parents and a foreign surrogate. Just <a href="https://npesu.unsw.edu.au/sites/default/files/npesu/data_collection/Assisted%20Reproductive%20Technology%20in%20Australia%20and%20New%20Zealand%202021.pdf">100</a> surrogacy births were reported by Australian and New Zealand fertility clinics in 2021. </p>
<p>Australian surrogacy laws and <a href="https://www.nhmrc.gov.au/about-us/publications/art">ethical guidelines</a> aim to protect the interest of everyone involved with surrogacy. They also recognise the most important consideration of all is the welfare of the children born.</p>
<p>However, children born through international surrogacy aren’t protected by Australian laws because they’re born overseas. Our new <a href="https://doi.org/10.1080/14647273.2023.2270157">research</a> shows this can increase the physical and psychological risks to the child. </p>
<p>Making surrogacy easier to access in Australia could protect future children born through surrogacy. </p>
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<strong>
Read more:
<a href="https://theconversation.com/becoming-a-parent-through-surrogacy-can-have-ethical-challenges-but-it-is-a-positive-experience-for-some-167760">Becoming a parent through surrogacy can have ethical challenges – but it is a positive experience for some</a>
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<h2>How does international surrogacy impact children?</h2>
<p>We surveyed more than 300 Australians who were parents through surrogacy, or were planning on having a child via surrogacy. We asked if they had picked international or domestic surrogacy and why, and we asked about the fertility treatment they and their surrogate received.</p>
<p>Respondents who had a child through international surrogacy commonly reported using two fertility treatments currently <a href="https://www.nhmrc.gov.au/about-us/publications/art">banned</a> in Australia: multiple embryo transfer and anonymous egg donation. </p>
<p>Surrogates sometimes supply their own egg, but mostly the egg is supplied from one of the intended parents or a donor. Once the egg has been fertilised, the resulting embryo is then transferred to the surrogate.</p>
<p>In Australia, only one embryo can be transferred to surrogates at a time. This is because multiple embryo transfer <a href="https://doi.org/10.1080/14647273.2020.1785643">increases the risk</a> of twin, or even triplet, pregnancies. These pregnancies are linked to higher rates of complications for the pregnant woman and the baby, including preterm birth. Preterm birth is when a baby is born before 37 weeks of pregnancy and is the <a href="https://www.who.int/en/news-room/fact-sheets/detail/preterm-birth">main cause</a> of death in children below the age of five.</p>
<figure class="align-center ">
<img alt="Dad carries his twins in a field, while an older child runs ahead" src="https://images.theconversation.com/files/532123/original/file-20230615-27-5yx7gk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/532123/original/file-20230615-27-5yx7gk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/532123/original/file-20230615-27-5yx7gk.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/532123/original/file-20230615-27-5yx7gk.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/532123/original/file-20230615-27-5yx7gk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/532123/original/file-20230615-27-5yx7gk.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/532123/original/file-20230615-27-5yx7gk.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">Multiple births are more likely with international surrogacy.</span>
<span class="attribution"><a class="source" href="https://unsplash.com/photos/O-RKu3Aqnsw">Unsplash/Juliane Liebermann</a></span>
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<p>Of the survey respondents who had completed international surrogacy, 37% reported multiple embryos had been transferred to their surrogate. Some 27% of parents through international surrogacy had a preterm baby and 11% had twins or triplets. In contrast, just 11% of parents through surrogacy in Australia had a preterm baby and none had twins or triplets. </p>
<p>If a donor egg is used in Australia, the donor-conceived person can access information about their donor once they turn 18. Anonymous donation is <a href="https://www.nhmrc.gov.au/about-us/publications/art">not allowed</a> because research shows many people born through egg or sperm donation <a href="https://pubmed.ncbi.nlm.nih.gov/22908619/">want to know</a> the identity of their donor. </p>
<p>Of the respondents who used donor eggs in international surrogacy, 47% said the identity of the donor was anonymous. Australians born through international surrogacy with anonymous egg donors may never know who their genetic mother is.</p>
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<p>
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Read more:
<a href="https://theconversation.com/who-are-my-parents-why-new-zealands-creaky-surrogacy-laws-are-overdue-for-major-reform-166745">Who are my parents? Why New Zealand’s 'creaky' surrogacy laws are overdue for major reform</a>
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<h2>Why choose international surrogacy?</h2>
<p>The most popular reasons for picking international surrogacy were that surrogacy in Australia is long and complicated and it is difficult to find an Australian surrogate. </p>
<p>Most Australian surrogacy arrangements occur between <a href="https://sarahjefford.com/australian-surrogacy-statistics/">friends and family members</a>. If this is not possible, intended parents can join online communities to meet potential surrogates. However, the number of intended parents in these communities far <a href="https://www.surrogacyaustralia.org/surrogacy-process-chart/">outweighs</a> the number of surrogates.</p>
<p>For those lucky to find an Australian surrogate, they must complete a series of legal requirements as part of the arrangement. These requirements protect the interests of surrogacy participants and include legal advice, counselling and a court order to transfer parentage from the surrogate to the intended parents.</p>
<p>With international surrogacy, commercial agencies or brokers can match intended parents to a surrogate and the various Australian legal requirements may not be needed. </p>
<figure class="align-center ">
<img alt="Pregnant women holds her hands under her belly" src="https://images.theconversation.com/files/532122/original/file-20230615-23-whx9eu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/532122/original/file-20230615-23-whx9eu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/532122/original/file-20230615-23-whx9eu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/532122/original/file-20230615-23-whx9eu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/532122/original/file-20230615-23-whx9eu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/532122/original/file-20230615-23-whx9eu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/532122/original/file-20230615-23-whx9eu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">In some countries, you don’t have to go to court for legal parentage of babies born via surrogacy.</span>
<span class="attribution"><a class="source" href="https://unsplash.com/photos/WI-x1wo_Jm4">Unsplash/Omurden Cengiz</a></span>
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<h2>How can the risks of international surrogacy be reduced?</h2>
<p>Intended parents considering international surrogacy should choose single embryo transfers and, if required, a known donor. </p>
<p>However, intended parents may not always be equipped with the information or resources to make this choice. Known donors may not always be available overseas and some of our respondents said they transferred multiple embryos because they were following the advice of their doctor. </p>
<p>Most respondents said they would prefer to complete surrogacy in Australia if it were possible. This means if surrogacy was more accessible in Australia, fewer people might go overseas and more babies might be born in Australia where regulations protect the child’s physical and psychological health.</p>
<p>To make surrogacy more accessible, surrogacy laws should be reviewed by an inquiry by the Australian Law Reform Commission. The House of Representatives Standing Committee on Social Policy and Legal Affairs <a href="https://www.aph.gov.au/Parliamentary_Business/Committees/House/Social_Policy_and_Legal_Affairs/Inquiry_into_surrogacy">recommended</a> such an inquiry in 2016. This was never undertaken, but we don’t know why.</p>
<p>The Law Commission of England and Wales and the Scottish Law Commission <a href="https://www.lawcom.gov.uk/project/surrogacy/">recently published</a> recommendations for surrogacy law reform in the United Kingdom. One recommendation removes the need for a court to grant intended parents legal recognition. This is a <a href="https://www.lawgazette.co.uk/news/family-lawyers-welcome-revolutionary-surrogacy-recommendations/5115585.article">welcome</a> step forward. </p>
<p>However, the recommendations have also been <a href="https://www.progress.org.uk/law-commissions-proposals-wont-achieve-surrogacy-modernisation/">criticised</a> for not allowing surrogate compensation, which could discourage some people from becoming surrogates. Compensation is financial acknowledgement of the time and effort involved with the surrogacy pregnancy and is currently not allowed in Australia.</p>
<p>Law reform in Australia must address all barriers for domestic surrogacy, including the shortage of surrogates, to protect the welfare of children born through surrogacy.</p>
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<strong>
Read more:
<a href="https://theconversation.com/surrogacy-shake-up-in-uk-would-create-uneven-treatment-for-birth-mothers-202872">Surrogacy shake up in UK would create uneven treatment for birth mothers</a>
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</em>
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<img src="https://counter.theconversation.com/content/206182/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Ezra Kneebone receives funding from the Australian Government Department of Education. </span></em></p><p class="fine-print"><em><span>Karin Hammarberg works for the Victorian Assisted Reproductive Treatment Authority. </span></em></p><p class="fine-print"><em><span>Kiri Beilby does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Children born through international surrogacy aren’t protected by Australian laws.Ezra Kneebone, PhD Candidate, Monash UniversityKarin Hammarberg, Senior Research Fellow, Global and Women's Health, School of Public Health & Preventive Medicine, Monash UniversityKiri Beilby, Course Coordinator (Graduate Diploma of Reproductive Science), Monash UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2047302023-09-11T12:34:41Z2023-09-11T12:34:41ZCan animals give birth to twins?<figure><img src="https://images.theconversation.com/files/535718/original/file-20230705-25-ey6m80.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Some animals, including goats, regularly give birth to two babies at once.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/two-kid-goats-in-field-royalty-free-image/71919552?phrase=baby+goats&adppopup=true">Image Source via Getty Images</a></span></figcaption></figure><figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=293&fit=crop&dpr=1 600w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=293&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=293&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=368&fit=crop&dpr=1 754w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=368&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=368&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<p><em><a href="https://theconversation.com/us/topics/curious-kids-us-74795">Curious Kids</a> is a series for children of all ages. If you have a question you’d like an expert to answer, send it to <a href="mailto:curiouskidsus@theconversation.com">curiouskidsus@theconversation.com</a>.</em></p>
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<blockquote>
<p><strong>Can animals give birth to twins? – Mia C., age 10</strong></p>
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<hr>
<p>Ask any parent – welcoming a new baby to the family is exciting, but it comes with a lot of work. And when the new addition is a pair of babies – twins – parents really have their work cut out for them.</p>
<p>For many animal species it’s the norm to have multiple babies at once. A litter of <a href="https://www.researchgate.net/profile/Jean-Pierre-Bidanel/publication/249324578_An_overview_of_twenty_years_of_selection_for_litter_size_in_pigs_using_hyperprolific_schemes/links/54525ccd0cf2cf51647a125b/An-overview-of-twenty-years-of-selection-for-litter-size-in-pigs-using-hyperprolific-schemes.pdf">piglets can be as many as 11</a> or more! </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/538767/original/file-20230721-6326-u8u6fj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A mother pig lying on hay in a barn surrounded by multiple piglets" src="https://images.theconversation.com/files/538767/original/file-20230721-6326-u8u6fj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/538767/original/file-20230721-6326-u8u6fj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=489&fit=crop&dpr=1 600w, https://images.theconversation.com/files/538767/original/file-20230721-6326-u8u6fj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=489&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/538767/original/file-20230721-6326-u8u6fj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=489&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/538767/original/file-20230721-6326-u8u6fj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=615&fit=crop&dpr=1 754w, https://images.theconversation.com/files/538767/original/file-20230721-6326-u8u6fj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=615&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/538767/original/file-20230721-6326-u8u6fj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=615&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Pigs are one species that gives birth to large litters.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/mother-pig-with-piglets-royalty-free-image/164283190?phrase=pig+litter&adppopup=true">Arthur Dries/DigitalVision via Getty Images</a></span>
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<p><a href="https://www.vetmed.msstate.edu/directory/mhj95">We are</a> <a href="https://www.vetmed.msstate.edu/directory/tj1232">faculty members</a> at Mississippi State University College of Veterinary Medicine. We’ve been present for the births of many puppies and kittens over the years – and the animal moms almost always deliver multiples.</p>
<p>But are all those animal siblings who share the same birthday twins?</p>
<h2>Twins are two peas in a pod</h2>
<p><a href="https://www.britannica.com/science/multiple-birth">Twins are defined as two offspring</a> from the same pregnancy.</p>
<p>They can be identical, which means a single sperm fertilized a single egg that divided into two separate cells that went on to develop into two identical babies. They share the same DNA, and that’s why the two twins are essentially indistinguishable from each other.</p>
<p><a href="https://www.britannica.com/science/dizygotic-twin">Twins can also be fraternal</a>. That’s the outcome when two separate eggs are fertilized individually at the same time. Each twin has its own set of genes from the mother and the father. One can be male and one can be female. Fraternal twins are basically as similar as any set of siblings.</p>
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<a href="https://images.theconversation.com/files/538761/original/file-20230721-25-9rs8a7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="diagram of two sperm fertilizing two eggs yielding two embryos, and one sperm fertilizing one egg that divides into two separate embryos" src="https://images.theconversation.com/files/538761/original/file-20230721-25-9rs8a7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/538761/original/file-20230721-25-9rs8a7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=156&fit=crop&dpr=1 600w, https://images.theconversation.com/files/538761/original/file-20230721-25-9rs8a7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=156&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/538761/original/file-20230721-25-9rs8a7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=156&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/538761/original/file-20230721-25-9rs8a7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=196&fit=crop&dpr=1 754w, https://images.theconversation.com/files/538761/original/file-20230721-25-9rs8a7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=196&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/538761/original/file-20230721-25-9rs8a7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=196&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Fraternal twins originate in two eggs fertilized separately, while identical twins originate in a single fertilized egg that divides to create two embryos.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/illustration/dizygotic-and-monozygotic-twins-illustration-royalty-free-illustration/1042127200?phrase=fertilization+of+twins+identical+vs+fraternal+diagram&adppopup=true">Veronika Zakharova/Science Photo Library via Getty Images</a></span>
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<p>Approximately <a href="https://www.ncbi.nlm.nih.gov/books/NBK493200/#:%7E:text=Twin%20births%20account%20for%20approximately,percent%20of%20all%20twin%20gestations.">3% of human pregnancies in the United States produce twins</a>. Most of those are fraternal – approximately <a href="https://alphabiolabsusa.com/learning-center/identical-vs-fraternal-twins/">one out of every three pairs of twins</a> is identical.</p>
<h2>Multiple babies from one animal mom</h2>
<p>Each kind of animal has its own standard number of offspring per birth. People tend to know the most about domesticated species that are kept as pets or farm animals.</p>
<p>One study that surveyed the <a href="https://doi.org/10.1016/j.theriogenology.2010.10.034">size of over 10,000 litters among purebred dogs</a> found that the average number of puppies varied by the size of the dog breed. Miniature breed dogs – like chihuahuas and toy poodles, generally weighing less than 10 pounds (4.5 kilograms) – averaged 3.5 puppies per litter. Giant breed dogs – like mastiffs and Great Danes, typically over 100 pounds (45 kilograms) – averaged more than seven puppies per litter.</p>
<p>When a litter of dogs, for instance, consists of only two offspring, people tend to refer to the two puppies as twins. Twins are the <a href="https://doi.org/10.4141/cjas95-022">most common pregnancy outcome in goats</a>, though mom goats can give birth to a single-born kid or larger litters, too. Sheep frequently have twins, but <a href="https://doi.org/10.1016/j.smallrumres.2013.05.018">single-born lambs are more common</a>.</p>
<p>Horses, which are pregnant for 11 to 12 months, and cows, which are pregnant for nine to 10 months, tend to have just one foal or calf at a time – but twins may occur. Veterinarians and ranchers have long believed that it would be financially beneficial to encourage the conception of twins in dairy and beef cattle. Basically the farmer would get two calves for the price of one pregnancy.</p>
<p>But twins in cattle may result in <a href="https://doi.org/10.2527/jas.2007-0210">birth complications for the cow and undersized calves</a> with reduced survival rates. Similar risks come with <a href="https://doi.org/10.1016/j.theriogenology.2021.09.018">twin pregnancies in horses</a>, which tend to lead to both pregnancy complications that may harm the mare and the birth of weak foals.</p>
<h2>DNA holds the answer to what kind of twins</h2>
<p>So plenty of animals can give birth to twins. A more complicated question is whether two animal babies born together are identical or fraternal twins.</p>
<p>Female dogs and cats <a href="https://doi.org/10.1146/annurev.es.04.110173.001035">ovulate multiple eggs at one time</a>. Fertilization of individual eggs by distinct spermatazoa from a male produces multiple embryos. This process results in puppies or kittens that are fraternal, not identical, even though they may look very much the same.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/538776/original/file-20230721-6732-s8he64.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A mother cat lying down on a blanket and nursing her kittens." src="https://images.theconversation.com/files/538776/original/file-20230721-6732-s8he64.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/538776/original/file-20230721-6732-s8he64.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/538776/original/file-20230721-6732-s8he64.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/538776/original/file-20230721-6732-s8he64.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/538776/original/file-20230721-6732-s8he64.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/538776/original/file-20230721-6732-s8he64.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/538776/original/file-20230721-6732-s8he64.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Though born from the same litter, these kittens all have different sets of genes.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/mother-cat-nursing-baby-kittens-royalty-free-image/1070428270?phrase=litter+of+kittens&adppopup=true">bozhdb/iStock via Getty Images</a></span>
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<p>Biologists believe that identical twins in most animals are very rare. The tricky part is that lots of animal siblings look very, very similar and researchers need to do a DNA test to confirm whether two animals do in fact share all their genes. Only <a href="https://doi.org/10.1111/rda.12746">one documented report</a> of identical twin dogs was confirmed by DNA testing. But no one knows for sure how frequently fertilized animal eggs split and grow into identical twin animal babies.</p>
<p>And reproduction is different in various animals. For instance, nine-banded armadillos normally <a href="https://doi.org/10.1656/1528-7092(2002)001%5B0287:PVWABL%5D2.0.CO;2">give birth to identical quadruplets</a>. After a mother armadillo releases an egg and it becomes fertilized, it splits into four separate identical cells that develop into identical pups. Its relative, the seven-banded armadillo, can give birth to anywhere from <a href="https://doi.org/10.7717/peerj.3650">seven to nine identical pups at one time</a>.</p>
<p>There’s still a lot that scientists aren’t sure about when it comes to twins in other species. Since DNA testing is not commonly performed in animals, no one really knows how often identical twins are born. It’s possible – maybe even likely – that identical twins may have been born in some species without anyone’s ever knowing.</p>
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<p><em>Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to <a href="mailto:curiouskidsus@theconversation.com">CuriousKidsUS@theconversation.com</a>. Please tell us your name, age and the city where you live.</em></p>
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<p class="fine-print"><em><span>The authors do not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Animals often give birth to litters of more than one offspring at a time. But are those babies twins?Michael Jaffe, Associate Professor of Small Animal Surgery, Mississippi State UniversityTracy Jaffe, Assistant Clinical Professor of Veterinary Medicine, Mississippi State UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1721452021-12-29T21:26:56Z2021-12-29T21:26:56Z5 things research from twins taught us about health, behaviour and what makes us unique<figure><img src="https://images.theconversation.com/files/436555/original/file-20211209-140109-1g6swwj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://unsplash.com/photos/M-adWhDQd7Y">Keisha Montfleury/Unsplash</a></span></figcaption></figure><p>Researchers often compare the differences between identical and fraternal twins to better understand health and behaviour. </p>
<p>The first major insight is that genes and environments almost always combine to influence our life trajectory. Sometimes the largest factor is genetics (think genetic disorders). Sometimes it’s environment (think infections). Mostly, it’s somewhere in between. </p>
<p>Such studies have accelerated the search for genes and environmental agents that cause or trigger diseases. This has helped us understand, treat and even prevent diseases. As twin research has <a href="https://www.twins.org.au/research/tools-and-resources/125-conversation-in-twin-research/377-twin-research-designs-and-analytic-approaches">matured</a>, it has progressed to addressing important questions about when and how diseases originate. </p>
<p>So what has research from twins taught us about specific diseases and the human body?</p>
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<p>
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<strong>
Read more:
<a href="https://theconversation.com/seeing-double-why-twins-are-so-important-for-health-and-medical-research-5273">Seeing double: why twins are so important for health and medical research</a>
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<h2>1. Smoking increases the risk of bone fracture</h2>
<p>Most studies linking environment and disease are complicated by genetic factors. To get around this, we can work with twins who differ in environmental factors. </p>
<p>One such <a href="https://www.nejm.org/doi/full/10.1056/nejm199402103300603">Australian study from 1994</a> compared 20 pairs of female twins in which only one of each pair was a long-term, heavy smoker. </p>
<p>The researchers found smoking one pack of cigarettes a day for 20 years resulted in sufficient loss in bone density to cause osteoporosis. This doubled the risk of having a bone fracture. </p>
<p>This provided compelling evidence that smoking causes osteoporosis and an increased risk of bone fractures.</p>
<h2>2. Events around the time of birth are not a major cause of epilepsy</h2>
<p>Epilepsy is a group of disorders where brain activity is abnormal and seizures are the presenting feature. Traditionally, diagnosis was not possible until after a person’s first seizure, which can occur at any stage of life, from babies to the elderly.</p>
<p><a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4166361/">Twin studies</a> since the 1960s have shown a mix of genes and environment cause epilepsy. However, until the early 1990s, it was assumed that problems during the birthing process were a major cause of epilepsy.</p>
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<img alt="Older men stand beside each other, smiling." src="https://images.theconversation.com/files/436563/original/file-20211209-68670-15wadz1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/436563/original/file-20211209-68670-15wadz1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/436563/original/file-20211209-68670-15wadz1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/436563/original/file-20211209-68670-15wadz1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/436563/original/file-20211209-68670-15wadz1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/436563/original/file-20211209-68670-15wadz1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/436563/original/file-20211209-68670-15wadz1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">Identical twins share almost all their DNA.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/senior-twin-men-smiling-on-camera-1919843804">Shutterstock</a></span>
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<p>Obstetricians and midwives were often blamed for causing epilepsy. However, a <a href="https://pubmed.ncbi.nlm.nih.gov/8255449/">twin study</a> in 1993 did not support a link between minor problems during birth and the later development of epilepsy. </p>
<p>This information has helped doctors and their patients better understand the causes of epilepsy and not necessarily attribute blame to the birthing process.</p>
<h2>3. Identical twins are different under the skin from before birth</h2>
<p>Genetically identical twins nearly always look identical. Yet, at birth, they have already accumulated differences in the structure and function of their genes. </p>
<p>These <a href="https://press.princeton.edu/books/hardcover/9780691173887/innate">differences</a> are caused by a mix of chance events and individual experiences in the womb.</p>
<p>The location a fertilised egg implants in the womb is random, but some locations are more favourable to growth. For the subset of identical twins who split before they reach the womb, different locations could create different environments in which a baby develops. </p>
<figure class="align-center ">
<img alt="Twin newborn babies sleep, their arms raised." src="https://images.theconversation.com/files/436558/original/file-20211209-142574-qu4qd5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/436558/original/file-20211209-142574-qu4qd5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=305&fit=crop&dpr=1 600w, https://images.theconversation.com/files/436558/original/file-20211209-142574-qu4qd5.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=305&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/436558/original/file-20211209-142574-qu4qd5.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=305&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/436558/original/file-20211209-142574-qu4qd5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=384&fit=crop&dpr=1 754w, https://images.theconversation.com/files/436558/original/file-20211209-142574-qu4qd5.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=384&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/436558/original/file-20211209-142574-qu4qd5.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=384&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">Identical, but still different.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/sleeping-newborn-identical-boy-twins-267663011">Shutterstock</a></span>
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<p>As a result of this or other chance events, around one in six twins differ more than 20% in weight at birth, which may be associated with an increased <a href="https://obgyn.onlinelibrary.wiley.com/doi/10.1111/aogs.13613">risk</a> of illness at birth, especially for the smaller twin. </p>
<p>Such individual experiences could also help explain Brazilian twin pairs in which only one child was born with <a href="https://www.mdpi.com/2414-6366/5/4/188">Zika virus</a> infection.</p>
<h2>4. Leukaemia originates before birth</h2>
<p>Changes in the genetic sequence of blood cells can predispose people to develop leukaemia (cancer of the blood). </p>
<p>Such changes are unique to each person but <em>when</em> these changes happened to people used to be a mystery. That was until identical twin children were <a href="https://www.sciencedirect.com/science/article/pii/S0006497120768072?via%3Dihub">discovered with leukaemias</a> originating from the same cell. </p>
<p>Lymphocytes (white blood cells) of the immune system shuffle their immune genes at random, making each person genetically unique, even identical twins. </p>
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Read more:
<a href="https://theconversation.com/same-same-but-different-when-identical-twins-are-non-identical-112684">Same same but different: when identical twins are non-identical</a>
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<p>The researchers <a href="https://www.sciencedirect.com/science/article/pii/S0006497120768072?via%3Dihub">concluded</a> the leukaemia started in one twin in the womb and spread to the other twin through blood vessels in a shared placenta. </p>
<p>But while the first step towards leukaemia happened before birth, the cancer progression differed among the twins, resulting in leukaemia being diagnosed at different ages.</p>
<p>This provided the first evidence that some leukaemias can lay dormant for years and enabled future research that would pinpoint the events along this process. </p>
<h2>5. Many twins don’t know if they’re identical or fraternal</h2>
<p>Identical twins start as one fertilised egg that splits after a few days. They share almost 100% of their DNA and are almost always the same sex. </p>
<p>Fraternal twins result from two eggs fertilised around the same time. They’re as genetically different as any pair of siblings and can have the same, or different sex. </p>
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<img alt="Twin women hug outside in the sunshine." src="https://images.theconversation.com/files/437106/original/file-20211213-25-1fbing6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/437106/original/file-20211213-25-1fbing6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/437106/original/file-20211213-25-1fbing6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/437106/original/file-20211213-25-1fbing6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/437106/original/file-20211213-25-1fbing6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/437106/original/file-20211213-25-1fbing6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/437106/original/file-20211213-25-1fbing6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">Fraternal twins are as genetically different as a pair of siblings.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/portrait-identical-ginger-twin-sisters-smiling-723502963">Shutterstock</a></span>
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<p>In 2012, my colleagues and I at <a href="https://www.twins.org.au/">Twins Research Australia</a> conducted a study at a national twins festival on pairs who had any uncertainty about their genetic identity. We used “genetic fingerprinting” on DNA from cheek swabs provided by same-sex twins of all ages. This test is the definitive way of discovering whether twins are identical or fraternal. </p>
<p>We compared this with perceptions of the twins themselves before they took the test. </p>
<p>We <a href="https://www.twins.org.au/research/tools-and-resources/125-conversation-in-twin-research/374-the-importance-of-zygosity-knowledge-for-twins-and-science">found</a> almost one-third of the twins we tested had been either incorrect or unsure about their genetic identity. Some had even been misinformed by medical professionals. </p>
<p>The universal sentiment was twins and their families felt better knowing the truth. Our data enabled us to develop better educational <a href="https://www.twins.org.au/twins-and-families/expecting-twins">resources</a> for twins and their advocates to know more about themselves. </p>
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<em>
<strong>
Read more:
<a href="https://theconversation.com/not-all-twins-are-identical-and-thats-been-an-evolutionary-puzzle-until-now-138209">Not all twins are identical and that's been an evolutionary puzzle, until now</a>
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<img src="https://counter.theconversation.com/content/172145/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jeffrey Craig is Deputy Director of Twins Research Australia and President of the International Society for Twin Studies. He receives funding from the IMPACT Institute, Deakin University, the National Health and Medical Research Council, Australia, and the Waterloo Foundation, UK. He is affiliated with the Gene(e)quality Network. </span></em></p>Genes and environments almost always combine to influence our risk of diseases. Research in twins has helped us understand how.Jeffrey Craig, Professor in Medical Sciences, Deakin UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1402882020-06-10T12:05:48Z2020-06-10T12:05:48ZHow sensitive you are may be partly down to your genes – new research<figure><img src="https://images.theconversation.com/files/340886/original/file-20200610-34710-kq86y7.jpg?ixlib=rb-1.1.0&rect=6%2C25%2C4243%2C2796&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">By looking at twins, we were able to see whether sensitivity is an inherited trait.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/father-carrying-twin-babies-car-seats-1128705452">Zoia Kostina/ Shutterstock</a></span></figcaption></figure><p>People differ substantially in how much they’re affected by experiences in their lives. Some people seem to be more affected by daily stress, or the loss of someone close to them. On the other hand, some people seem to get through the same experiences relatively unscathed. Similarly, some people benefit strongly from counselling, or having a support system of close family and friends. Others seem better able to manage on their own.</p>
<p>But understanding why some people are more sensitive than others isn’t just a question of how they were raised, and the experiences they’ve been through. In fact, previous research has found that some people in general seem <a href="https://content.apa.org/doiLanding?doi=10.1037%2Fa0017376">more sensitive</a> to what they experience – and some are generally less sensitive.</p>
<p>According to psychological theories, differences in such “<a href="https://srcd.onlinelibrary.wiley.com/doi/abs/10.1111/cdep.12120">environmental sensitivity</a>” reflect how able a person is to perceive and process information about the events in their life. Essentially, this is how sensitive you are. A person’s sensitivity level can be measured by a short questionnaire.</p>
<p>But many also believe that sensitivity has a genetic basis. Previous studies have investigated whether <a href="https://www.nature.com/articles/mp200944">specific genes</a> make some people more sensitive. However, no study so far has provided an estimate for how much genetic factors contribute to how sensitive a person is. </p>
<p>In our <a href="https://www.nature.com/articles/s41380-020-0783-8">latest research</a>, we looked at data collected from more than 2,800 identical and non-identical twins from the UK who have taken part in the <a href="http://www.teds.ac.uk/about-teds">Twins Early Development Study</a> in order to investigate the heritability of sensitivity and how it relates to other personality traits. Heritability describes what proportion of the differences between people in relation to a particular trait can be attributed to genetic factors alone.</p>
<p>Participants were asked to <a href="https://psycnet.apa.org/record/2017-42386-001">fill out a questionnaire</a> that measured their sensitivity levels. The questionnaire asked them how much they usually get affected by a various psychological and sensory experiences. For example, they were asked how much they notice when small things around them have changed, whether loud noises make them feel uncomfortable, and whether they dislike watching violent TV programmes. The participants were about 17 years old when the data were collected. </p>
<p>Comparing the similarity of identical with non-identical twin pairs allowed us to estimate the degree to which a trait is inherited. Identical twins share the same genes, but non-identical twins only share half of their genes. However, both types of twins share the same environment. If siblings in identical twin pairs are more similar to each other than non-identical twins, this indicates the existence of a genetic basis for the examined trait. Twin studies have long been used to estimate heritability.</p>
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<img alt="" src="https://images.theconversation.com/files/340887/original/file-20200610-34678-1d3a0zb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/340887/original/file-20200610-34678-1d3a0zb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/340887/original/file-20200610-34678-1d3a0zb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/340887/original/file-20200610-34678-1d3a0zb.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/340887/original/file-20200610-34678-1d3a0zb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/340887/original/file-20200610-34678-1d3a0zb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/340887/original/file-20200610-34678-1d3a0zb.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">Sensitive people can thank their genes.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/supportive-african-husband-embracing-crying-wife-1283870137">fizkes/ Shutterstock</a></span>
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<p>However, these types of studies have also been <a href="https://www.sciencedirect.com/science/article/abs/pii/S0049089X13001397?via%3Dihub">criticised for their limitations</a>. For example, identical twins’ experiences, such as how they’re raised, or the environment they’re raised in, may be different than that of non-identical twins, which can lead to an overestimation of heritability estimates. This could mean that the heritability is not as high as sometimes indicated when using this type of research method.</p>
<h2>Genetics versus environment</h2>
<p>We found that 47% of the differences in peoples’ sensitivity are indeed explained by genetic factors. But the remaining 53% of a person’s level of sensitivity is shaped by life experiences. In other words, genetics accounts for just under half of the reason you may be a more sensitive person.</p>
<p>Importantly, our research examined the genetic basis of sensitivity in more detail. We wondered whether sensitivity was made up of one genetic component or multiple ones. We found that sensitivity is composed of multiple genetic components that together make up a person’s specific shape of sensitivity. </p>
<p>Besides a general genetic component for sensitivity, we also found a component associated with sensitivity to negative experiences and a component that reflects sensitivity to more positive experiences. This means that genetics influence why some people may be more sensitive in general, while others may be more sensitive to either the positive or negative things that happen to them.</p>
<p>Finally, we also investigated how inherited genetic sensitivity relates to other common personality traits: openness, conscientiousness, agreeableness, extraversion and neuroticism. We found that the genetic component of sensitivity was related to neuroticism and extraversion but not with any of the other personality traits. </p>
<p>Although this study didn’t examine what the specific genes are that make some people more sensitive than others, it did show that sensitivity is a heritable trait with a substantial genetic basis. We also found that a large part of this heritability is shared with other common personality traits, such as neuroticism and extraversion.</p>
<p>Future studies on sensitivity may look at identifying the specific genes that underlie this complex trait. Uncovering how genes shape sensitivity is important in our understanding of why some individuals fare worse in response to traumas and stressors – and why some benefit more from interventions aimed at promoting resilience and health.</p><img src="https://counter.theconversation.com/content/140288/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Elham Assary receives funding from The Wellcome Trust.</span></em></p><p class="fine-print"><em><span>Michael Pluess receives funding from Jacobs Foundation, National Institutes of Health, and Elrha. </span></em></p>Our study looked at both identical and non-identical sets of twins. We found that how sensitive you are is actually inherited genetically.Elham Assary, Postdoctoral researcher, psychopathology and resilience, Queen Mary University of LondonMichael Pluess, Professor of Psychology, Queen Mary University of LondonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1382092020-05-11T19:58:39Z2020-05-11T19:58:39ZNot all twins are identical and that’s been an evolutionary puzzle, until now<figure><img src="https://images.theconversation.com/files/333912/original/file-20200511-49565-1bwrkd6.jpg?ixlib=rb-1.1.0&rect=0%2C98%2C3000%2C1904&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Beth Shepherd Peters/Shutterstock</span></span></figcaption></figure><p>When a mother gives birth to twins, the offspring are not always identical or even the same gender. Known as fraternal twins, they represent a longstanding evolutionary puzzle. </p>
<p>Identical twins arise from a single fertilised egg that accidentally splits in two, but fraternal twins arise when two eggs are released and fertilised. Why this would happen was the puzzle.</p>
<p>In research <a href="https://www.nature.com/articles/s41559-020-1173-y" title="An age-dependent ovulatory strategy explains the evolution of dizygotic twinning in humans">published today in Nature Ecology & Evolution</a> we used computer simulations and modelling to try to explain why natural selection favours releasing two eggs, despite the low survival of twins and the risks of twin births for mothers.</p>
<h2>Why twins?</h2>
<p>Since Michael Bulmer’s landmark 1970 <a href="https://books.google.com.au/books/about/The_Biology_of_Twinning_in_Man.html?id=awo-AAAAYAAJ">book on the biology of twinning in humans</a>, biologists have questioned whether double ovulation was favoured by natural selection or, like identical twins, was the result of an accident.</p>
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Read more:
<a href="https://theconversation.com/curious-kids-why-are-some-twins-identical-and-some-not-121435">Curious Kids: why are some twins identical and some not?</a>
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<p>At first glance, this seems unlikely. The embryo splitting that produces identical twins is not heritable and the incidence of identical twinning does not vary with other aspects of human biology. It seems accidental in every sense of the word. </p>
<p>In contrast, the incidence of fraternal twinning changes with <a href="https://doi.org/10.1017/S0021932000007896" title="Ethnic differences in twinning rates in Nigeria">maternal age</a> and is <a href="https://doi.org/10.1016/S0140-6736(03)14237-7" title="Twinning">heritable</a>.</p>
<p>Those do not sound like the characteristics of something accidental.</p>
<h2>The twin disadvantage</h2>
<p>In human populations without access to medical care there seems little benefit to having twins. <a href="https://doi.org/10.1046/j.1420-9101.2001.00287.x" title="The fitness of twin mothers: evidence from rural Gambia">Twins</a> are more likely to die in childhood than single births. Mothers of twins also have an increased risk of dying in childbirth.</p>
<p>In common with other great apes, women seem to be built to give birth to <a href="https://doi.org/10.1111/j.1558-5646.1990.tb05211.x" title="ON THE EVOLUTION OF HUMAN BROOD SIZE">one child at a time</a>. So if twinning is costly, why has evolution not removed it?</p>
<p>Paradoxically, in high-fertility populations, the mothers of twins often have <a href="https://doi.org/10.1046/j.1420-9101.2001.00287.x" title="The fitness of twin mothers: evidence from rural Gambia">more offspring</a> by the end of their lives than other mothers. This suggests having twins might have an evolutionary benefit, at least for mothers. </p>
<p>But, if this is the case, why are twins so rare?</p>
<h2>Modelling mothers</h2>
<p>To resolve these questions, together with colleagues Bob Black and Rick Smock, we <a href="https://doi.org/10.1038/s41559-020-1173-y" title="An age-dependent ovulatory strategy explains the evolution of dizygotic twinning in humans">constructed simulations and mathematical models</a> fed with data on maternal, child and fetal survival from real populations. </p>
<p>This allowed us to do something otherwise impossible: control in the simulations and modelling whether women ovulated one or two eggs during their cycles. We also modelled different <a href="https://doi.org/10.1016/j.tree.2007.09.002" title="The status of the conditional evolutionarily stable strategy">strategies</a>, where we switched women from ovulating one egg to ovulating two at different ages.</p>
<p>We could then compare the number of surviving children for women with different patterns of ovulation.</p>
<p>Women who switched from single to double ovulation in their mid-20s had the most children survive in our models – more than those who always released a single egg, or always released two eggs. </p>
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<a href="https://images.theconversation.com/files/333966/original/file-20200511-49558-4l8o82.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/333966/original/file-20200511-49558-4l8o82.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/333966/original/file-20200511-49558-4l8o82.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=374&fit=crop&dpr=1 600w, https://images.theconversation.com/files/333966/original/file-20200511-49558-4l8o82.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=374&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/333966/original/file-20200511-49558-4l8o82.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=374&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/333966/original/file-20200511-49558-4l8o82.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=470&fit=crop&dpr=1 754w, https://images.theconversation.com/files/333966/original/file-20200511-49558-4l8o82.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=470&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/333966/original/file-20200511-49558-4l8o82.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=470&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="attribution"><span class="license">Author provided</span></span>
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<p>This suggests natural selection favours an unconscious switch from single to double ovulation with increasing age.</p>
<h2>A strategy for prolonging fertility</h2>
<p>The reason a switch is beneficial is fetal survival – the chance that a fertilised egg will result in a liveborn child – <a href="https://doi.org/10.1126/science.aaa3337" title="Common variants spanning PLK4 are associated with mitotic-origin aneuploidy in human embryos">decreases rapidly as women age</a> </p>
<p>So switching to releasing two eggs increases the chance at least one will result in a successful birth.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/same-same-but-different-when-identical-twins-are-non-identical-112684">Same same but different: when identical twins are non-identical</a>
</strong>
</em>
</p>
<hr>
<p>But what about twinning? Is it a side effect of selection favouring fertility in older women? To answer this question, we ran the simulations again, except now when women double ovulated the simulation removed one offspring before birth.</p>
<p>In these simulations, women who double ovulated throughout their lives, but never gave birth to twins, had more children survive than those who did have twins and switched from single to double ovulating.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/333982/original/file-20200511-49558-d6tomg.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/333982/original/file-20200511-49558-d6tomg.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/333982/original/file-20200511-49558-d6tomg.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=374&fit=crop&dpr=1 600w, https://images.theconversation.com/files/333982/original/file-20200511-49558-d6tomg.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=374&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/333982/original/file-20200511-49558-d6tomg.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=374&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/333982/original/file-20200511-49558-d6tomg.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=470&fit=crop&dpr=1 754w, https://images.theconversation.com/files/333982/original/file-20200511-49558-d6tomg.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=470&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/333982/original/file-20200511-49558-d6tomg.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=470&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption"></span>
<span class="attribution"><span class="license">Author provided</span></span>
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</figure>
<p>This suggests the ideal strategy would be to always double ovulate but never produce twins, so fraternal twins are an accidental side effect of a beneficial strategy of double ovulating.</p><img src="https://counter.theconversation.com/content/138209/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Joseph L Tomkins receives funding from The Australian Research Council. </span></em></p><p class="fine-print"><em><span>Rebecca Sear and Wade Hazel do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.</span></em></p>While identical twins are seen more as an accidental splitting of a single egg, there could be a good reason mothers produce non-identical twins from two separate eggs.Joseph L Tomkins, Associate Professor in Evolutionary Biology, The University of Western AustraliaRebecca Sear, Head of the Department of Population Health, London School of Hygiene & Tropical MedicineWade Hazel, Professor of Biology, DePauw UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1214352019-09-16T20:38:52Z2019-09-16T20:38:52ZCurious Kids: why are some twins identical and some not?<figure><img src="https://images.theconversation.com/files/286866/original/file-20190805-117871-cfa7xg.jpg?ixlib=rb-1.1.0&rect=58%2C0%2C6500%2C4320&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Identical twins look the same, are the same sex, share the same birthday and shares the same genes. </span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/download/success?u=http%3A%2F%2Fdownload.shutterstock.com%2Fgatekeeper%2FW3siZSI6MTU2NTAwNDA1NiwiYyI6Il9waG90b19zZXNzaW9uX2lkIiwiZGMiOiJpZGxfNzUwOTcwMDU3IiwiayI6InBob3RvLzc1MDk3MDA1Ny9odWdlLmpwZyIsIm0iOjEsImQiOiJzaHV0dGVyc3RvY2stbWVkaWEifSwiaVFIeGdBL2NPMkQrVG0wSm9RNW9xZTBHQkJnIl0%2Fshutterstock_750970057.jpg&pi=41133566&m=750970057&src=fi5P8G5angPStHSzIyE-Ig-1-8">www.shuttershock.com </a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=293&fit=crop&dpr=1 600w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=293&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=293&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=368&fit=crop&dpr=1 754w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=368&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/281719/original/file-20190628-76743-26slbc.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=368&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<p><em>If you have a question you’d like an expert to answer, send it to curiouskids@theconversation.edu.au.</em> </p>
<hr>
<blockquote>
<p><strong>Why are some twins identical and some not? - Chloe, age 12, Australia</strong></p>
</blockquote>
<p>We have spent many years explaining how genes work to people who would like to have children, so we’re happy to answer this excellent question.</p>
<p>There are two types of twins: fraternal and identical.</p>
<p>Fraternal twins may be born on the same day but are not genetically the same. They look different, have different genes and may be of the same sex or the opposite sex. </p>
<p>Identical twins, on the other hand, look the same, share the same birthday and share the same genes. They are the same sex, meaning they will both be girls or they will both be boys.</p>
<p>To understand why, we need to look at what happens at the time a pregnancy starts. We call the start of a pregnancy the time of conception. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/curious-kids-why-do-people-grow-to-certain-sizes-105131">Curious Kids: Why do people grow to certain sizes?</a>
</strong>
</em>
</p>
<hr>
<h2>What happens when a woman becomes pregnant?</h2>
<p>Most women produce one egg each month. Each time a man and woman have sex, the man produces thousands of sperm. If the man and woman have sex and do not use contraception (for example a condom, IUD or the Pill), there is a chance that a sperm will fertilise the egg and the woman will become pregnant. </p>
<p>Most of the time, a single egg is fertilised, and goes on to develop into a single baby. If the egg is not fertilised, the woman will soon have her period, which is the way a woman’s body prepares itself for a new egg to be fertilised the next month.</p>
<p>We call a fertilised egg a “zygote”. This is a good word to remember, as we use it to help us understand the different ways identical twins develop during pregnancy (and knowing a word like zygote might impress your science teacher one day). </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/286867/original/file-20190805-117866-1r3rlpo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/286867/original/file-20190805-117866-1r3rlpo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/286867/original/file-20190805-117866-1r3rlpo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/286867/original/file-20190805-117866-1r3rlpo.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/286867/original/file-20190805-117866-1r3rlpo.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/286867/original/file-20190805-117866-1r3rlpo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/286867/original/file-20190805-117866-1r3rlpo.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/286867/original/file-20190805-117866-1r3rlpo.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Identical twins have come from a single egg and a single sperm, so they share the same genes as each other.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/puzzledfrog/134646915/">Heather/flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">CC BY-NC-ND</a></span>
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<h2>How do identical twins happen?</h2>
<p>As you may know, genes are the instructions that tell our bodies how to develop and grow. They are like a recipe for creating each of us as unique individuals. </p>
<p>We have two copies of each of our genes: one from our biological mother and one from our biological father. That’s why we look like both our mother and our father.</p>
<p>Identical twins happen when a zygote splits into two in the first few days after conception. They have come from a single egg and a single sperm, so they share the same genes as each other. The reason the zygote splits is thought to be inherited, which may be why some families have a few sets of identical twins.</p>
<p>Because identical twins come from a single zygote that splits in two, they have exactly the same genes – exactly the same recipe. They will both have the same coloured eyes and hair, and will look the same. Identical twins are always the same sex too – they will both be girls or they will both be boys. </p>
<h2>How do non-identical twins happen?</h2>
<p>Identical twins are also called monozygous twins. This just means that they have come from the same, single zygote (mono means “one”). Non-identical twins are sometimes called dizygous twins (di means “two”, so dizygous means two zygotes). </p>
<p>Earlier on, we said that most women produce one egg each month. Occasionally, a women will produce more than one egg in a month. Non-identical twins happen when a woman produces two eggs (in the same month) and both eggs are fertilised by two different sperm. </p>
<p>Unlike identical twins, non-identical twins do not share the same genes as each other. They grow together and share the same birthday, but they are only as related as any other brothers and sisters. Non-identical twins could both be girls, or both be boys, or could be one girl and one boy twin.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/291661/original/file-20190910-109915-1gas6pb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/291661/original/file-20190910-109915-1gas6pb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/291661/original/file-20190910-109915-1gas6pb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=396&fit=crop&dpr=1 600w, https://images.theconversation.com/files/291661/original/file-20190910-109915-1gas6pb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=396&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/291661/original/file-20190910-109915-1gas6pb.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=396&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/291661/original/file-20190910-109915-1gas6pb.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=497&fit=crop&dpr=1 754w, https://images.theconversation.com/files/291661/original/file-20190910-109915-1gas6pb.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=497&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/291661/original/file-20190910-109915-1gas6pb.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=497&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Identical twins come from a single zygote that splits in two. Non-identical twins happen when a woman produces two eggs (in the same month) and both eggs are fertilised by two different sperm.</span>
<span class="attribution"><span class="source">Shutterstock</span></span>
</figcaption>
</figure>
<p>Interestingly, there are more non-identical twins in Australia now than there have been before. The number of twin pregnancies has grown over the past 30 years. This might be partly because women in countries like Australia are having children when they are older and the chance of a twin pregnancy increases as women get older (as they are more likely to produce more than one egg in the same month). </p>
<p>The chance of a twin pregnancy is also higher if a couple uses assisted reproductive technology to help them to become pregnant. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/curious-kids-how-do-babies-learn-to-talk-111613">Curious Kids: how do babies learn to talk?</a>
</strong>
</em>
</p>
<hr>
<p><em>Hello, curious kids! Have you got a question you’d like an expert to answer? Ask an adult to send your question to curiouskids@theconversation.edu.au</em></p><img src="https://counter.theconversation.com/content/121435/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Alison McEwen is the vice president of the Human Genetics Society of Australasia.</span></em></p><p class="fine-print"><em><span>Chris Jacobs does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Interestingly, there are more non-identical twins in Australia now than there have been before. The number of twin pregnancies has grown over the past 30 years.Alison McEwen, Head of Discipline of Genetic Counselling, Graduate School of Health, University of Technology SydneyChris Jacobs, Senior Lecturer, Graduate School of Health, University of Technology SydneyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1126842019-02-28T06:36:03Z2019-02-28T06:36:03ZSame same but different: when identical twins are non-identical<figure><img src="https://images.theconversation.com/files/261379/original/file-20190228-150702-1050098.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Semi-identical twins share all the DNA passed on from their mother, but only a portion from their father.</span> <span class="attribution"><span class="source">From shutterstock.com</span></span></figcaption></figure><p>We all know a set of twins; perhaps even a set of identical twins. In Australia, twins account for <a href="https://www.twins.org.au/twins-and-families/frequently-asked-questions/63-twin-figures/23-frequency-of-twin-births">about one in 80</a> births.</p>
<p>But in research from the University of New South Wales and the Queensland University of Technology <a href="https://www.nejm.org/doi/full/10.1056/NEJMoa1701313">published today</a> in the New England Journal of Medicine, we learn of a very unique set of Australian twins. </p>
<p>A boy and a girl from Brisbane, aged four, have been identified as only the second set of semi-identical, or sesquizygotic twins, in the world. They are the first to have been observed <em>in utero</em> (in the womb).</p>
<p>This extremely rare phenomenon is the result of two sperm fertilising the same egg.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/seeing-double-why-twins-are-so-important-for-health-and-medical-research-5273">Seeing double: why twins are so important for health and medical research</a>
</strong>
</em>
</p>
<hr>
<h2>How do twins occur?</h2>
<p>The <a href="https://www.twins.org.au/twins-and-families/frequently-asked-questions/63-twin-figures/23-frequency-of-twin-births">most common type of twins</a> are non-identical twins, which can be the same or different sexes. Non-identical twins are also known as fraternal twins or dizygotic twins (from two zygotes, what we call the earliest embryo when the egg and sperm fuse).</p>
<p>This type of twinning occurs when more than one egg is released from the ovary at ovulation (normally just a single egg is selected and released), and both of the eggs are fertilised by different sperm. These twins are no more genetically similar than siblings born years apart.</p>
<p>The rates of non-identical twins <a href="https://www.twins.org.au/twins-and-families/frequently-asked-questions/63-twin-figures/23-frequency-of-twin-births">differ between groups</a>: it’s about eight in 1,000 in caucasian populations, 16 in 1,000 in African populations, and four in 1,000 in people of Asian decent. This suggests there is a genetic component to non-identical twins.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/explainer-twins-triplets-quadruplets-and-more-9810">Explainer: twins, triplets, quadruplets and more</a>
</strong>
</em>
</p>
<hr>
<p>Identical twins (also known as monozygotic – originating from one zygote) are less common. They are the product of just one egg and one sperm, which originally form one embryo, but which break into two during the earliest stages of pregnancy. Scientists are continuing to explore how this occurs biologically, but the process remains a mystery.</p>
<p>The <a href="https://www.twins.org.au/twins-and-families/frequently-asked-questions/63-twin-figures/23-frequency-of-twin-births">rate of identical twins</a> is consistent across the globe at about four in 1,000 births. This suggests it’s probably just a random biological phenomena not influenced by genetics.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/261380/original/file-20190228-150698-1162p1m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/261380/original/file-20190228-150698-1162p1m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=626&fit=crop&dpr=1 600w, https://images.theconversation.com/files/261380/original/file-20190228-150698-1162p1m.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=626&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/261380/original/file-20190228-150698-1162p1m.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=626&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/261380/original/file-20190228-150698-1162p1m.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=787&fit=crop&dpr=1 754w, https://images.theconversation.com/files/261380/original/file-20190228-150698-1162p1m.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=787&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/261380/original/file-20190228-150698-1162p1m.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=787&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">There are three different genetic configurations possible in human twins.</span>
<span class="attribution"><span class="source">UNSW</span></span>
</figcaption>
</figure>
<p>Prior to today, there was only <a href="https://link.springer.com/article/10.1007%2Fs00439-006-0279-x">one reported case</a> of a third type of twins – semi-identical twins. </p>
<p>In 2006, twins in the US were identified as semi-identical twins when they were examined as infants for another medical condition. They too were the result of one egg fusing with two sperm.</p>
<p>But now we know the Queensland siblings are the second set of twins to fall into this mysterious and fascinating category.</p>
<h2>What are semi-identical twins?</h2>
<p>Scientists believe semi-identical twins are the result of one egg allowing two sperm in simultaneously (which has previously been thought to be non-viable, meaning a pregnancy would never occur). </p>
<p>In the case reported in today’s research, the pregnancy was identified as twins at six weeks. The ultrasound showed they shared a placenta, which is <a href="https://fetus.ucsf.edu/monochorionic-twins">common to 70% of identical twins</a>. </p>
<p>But at 14 weeks of pregnancy, tests revealed the twins were non-identical – one was a girl and the other a boy. Ordinarily non-identical twins would have their own placenta. They were an anomaly.</p>
<p>Thorough DNA testing has revealed they are identical on the maternal side (confirming they came from just one egg, like identical twins). But they are more like non-identical twins on the paternal side, sharing only a proportion of their father’s DNA.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/how-twins-like-the-scott-brothers-distinguish-themselves-in-battle-1040">How twins like the Scott brothers distinguish themselves in battle</a>
</strong>
</em>
</p>
<hr>
<p>Scientists understand biologically how the DNA of our mother and father mix to create an embryo (and subsequently a baby). On the day the egg and sperm meet in the fallopian tube, the DNA, packaged into chromosomes, divides equally into two, allowing the baby to inherit one copy of information from mum and one from dad.</p>
<p>When this doesn’t go to plan, a baby may get too many or too few chromosomes, resulting in genetic disorders like Down Syndrome (an extra copy of Chromosome 21). This may also result in a pregnancy which is not viable. </p>
<h2>We still don’t know a lot</h2>
<p>There is no scientific precedent for how one embryo manages to separate three sets of chromosomes, as is the case in semi-identical twins. This may remain a scientific mystery.</p>
<p>We have no idea how similar these twins are going to look, although the best guess is that they will be like every other set of non-identical, fraternal twins. </p>
<p>As they are just the second case of this type of twins ever identified, it’s hard to know if they will have a special connection beyond their extra shared DNA.</p>
<p>A search by researchers through huge databases of twins across the globe failed to find more semi-identical twins, suggesting this type of twinning is very rare.</p>
<p>Discoveries like this that teach us just how much there is still to learn about biology and health, and how fascinating the world of reproduction and pregnancy is.</p><img src="https://counter.theconversation.com/content/112684/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Hannah Brown does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Semi-identical or sesquizygotic twins occur when two sperm fertilise one egg. A boy and a girl in Brisbane are only the second set of semi-identical twins known in the world.Hannah Brown, Postdoctoral Research Affiliate, University of Adelaide; Chief Science Storyteller, South Australian Health & Medical Research InstituteLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1025202018-09-04T09:09:35Z2018-09-04T09:09:35ZGenes shown to influence how well children do throughout their time at school<figure><img src="https://images.theconversation.com/files/234631/original/file-20180903-41720-14jmscn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/home">Shutterstock.</a></span></figcaption></figure><p>Children differ widely in how well they do at school. In recent years, researchers have shown that around two-thirds of differences in school achievement <a href="http://journals.sagepub.com/doi/full/10.1177/0956797613486982">can be explained</a> by differences in children’s genes. </p>
<p>Genes have been shown to influence how well children do at <a href="https://europepmc.org/articles/pmc2784897">primary school</a>, at the end of <a href="https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0080341">compulsory education</a>, and even <a href="https://theconversation.com/the-same-genes-influence-exam-results-across-a-range-of-school-subjects-45059">in different subjects</a>. However, less is known about how genetic and environmental factors contribute to how well a child continues to do academically throughout their time at school. </p>
<p>To study this, we used a sample of over 6,000 pairs of twins who are part of the UK-representative <a href="https://www.teds.ac.uk/about-teds">Twins Early Development Study</a> and analysed their test scores from primary school to the end of compulsory education. Our new <a href="https://www.nature.com/articles/s41539-018-0030-0">research found</a> that the twins’ educational achievement was remarkably stable: children who do well in primary school also tend to perform well in GCSE exams, taken at the end of compulsory education. </p>
<p>Using twins allows us to estimate the proportion of differences that can be explained by genetic factors. Identical twins share 100% of their genes, while non-identical twins share on average 50% of the genes that differ between people, just like other siblings. If identical twins are more alike on a particular trait than non-identical twins, such as school achievement, we can infer that it is influenced by their genes. We can then estimate the heritability of that trait – or the proportion of differences that are down to the differences in children’s DNA sequence. </p>
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Read more:
<a href="https://theconversation.com/the-same-genes-influence-exam-results-across-a-range-of-school-subjects-45059">The same genes influence exam results across a range of school subjects</a>
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<p>We looked at what factors influenced stability in educational achievement – when grades in a standardised test remain similar between primary and secondary school. We found that about 70% of the stability in achievement is explained by genetic factors, while 25% is accounted for by the twins’ shared environment, such as growing up in the same family and attending the same school. The remaining 5% was explained by their non-shared environment, such as different friends or different teachers. </p>
<p>When there was a change in educational achievement – where grades increased or dropped between primary and secondary school – we found this was largely explained by those environmental factors that are not shared by twins. </p>
<p>It’s reasonable to assume that this substantial influence of genes on the continuity of children’s achievement during their time at school can be explained by intelligence. Yet we found the influence of genes remained substantial – at 60% – even after accounting for intelligence, which was measured using several verbal and nonverbal tests taken by the twins over the course of childhood and adolescence.</p>
<h2>Predicting achievement using DNA</h2>
<p>While twin study estimates such as this can tell us about traits within large groups of people, recent scientific advances are revealing more about the influence of genes on the individual. There has been considerable recent success in identifying genetic variants associated with <a href="https://www.nature.com/articles/nature17671">educational attainment</a> through what are called genome-wide association studies (GWAS). These studies pinpoint genetic markers associated with certain traits. However, each genetic marker explains a very small proportion (less than 0.1%) of the individual differences in school performance. </p>
<p>A more powerful method was recently developed that sums up thousands of the genetic markers found in the GWAS studies to instead calculate a genome-wide “polygenic score”. This score <a href="https://www.nature.com/articles/s41588-018-0147-3">is now being used</a>, with increasing levels of accuracy, to predict variance in a trait, such as school achievement, for people unrelated to each other. </p>
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Read more:
<a href="https://theconversation.com/your-genes-can-help-predict-how-well-youll-do-in-school-heres-how-we-cracked-it-62848">Your genes can help predict how well you'll do in school – here's how we cracked it</a>
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<p>As part of our new study, we used data from <a href="https://www.nature.com/articles/nature17671">previous GWAS analyses</a> to create a polygenic score for education attainment. We calculated a score for one of each pair of our 6,000 sets of twins (so that everybody in this part of the study was unrelated). This predicted whether they would do well across their time at school. These predictions ranged from accounting for 4% of the variance in educational attainment at the start of primary school, to 10% of the variance at GCSE levels. Our findings confirmed the results from the first part of our twins analysis – that the same genetic variants play a role in explaining why children differ in achievement at every stage in development.</p>
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<p><em><strong>To hear more about Twins studies and why they are so important for science, listen to the <a href="https://theconversation.com/anthill-26-twins-98271">Twins episode</a> of The Anthill podcast.</strong></em> </p>
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<p>Our findings, which suggest that genes influence how well a child will do across the length of their time at school, should provide additional motivation to identify children in need of interventions as early as possible, as problems are likely to remain throughout the school years. In the future, polygenic score prediction, together with the prediction of environmental risks – such as exposure to certain neighbourhood, family, and school characteristics – might provide a tool to identify children with educational problems very early in life. They could then be provided with individualised learning programmes. </p>
<p>For example, we could use DNA tests at birth to identify children at genetic risk for developing reading problems, and give them early intervention. As preventive interventions have greater chances of succeeding early in life, a great strength of polygenic scores is that they can predict at birth just as well as later in life, which could be of particular help for those children who are likely to struggle the most.</p><img src="https://counter.theconversation.com/content/102520/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>The authors do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.</span></em></p>New research is pinpointing how much genes influence the stability of educational achievement.Kaili Rimfeld, Postdoctoral Research Associate, King's College LondonMargherita Malanchini, Postdoctoral Fellow, The University of Texas at AustinLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/935332018-06-14T10:13:06Z2018-06-14T10:13:06ZSleep problems are influenced by our genes – but this doesn’t mean they can’t be fixed<figure><img src="https://images.theconversation.com/files/215233/original/file-20180417-163978-srkwwh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/young-woman-bed-covering-ears-pillow-635978966?src=0OrrtEmRoZ_uJLgTH4_HWQ-1-4">Andriano.cz/Shutterstock.com</a></span></figcaption></figure><p>Some people struggle greatly with sleeplessness, whereas others appear to be able to nod off effortlessly, regardless of the circumstances. Perhaps the most obvious explanation for differences between us in terms of our sleep is the environmental challenges that we face. An unrelenting stint at work, relationship difficulties or receiving bad news are just some of the many life challenges that can lead to sleepless nights.</p>
<p>It’s no surprise that stressful life events <a href="https://academic.oup.com/sleep/article/37/8/1295/2416809">are associated with disturbed sleep</a>. The way we respond to sleeplessness in terms of our <a href="https://www.sciencedirect.com/science/article/pii/S0005796701000614">thoughts and behaviours</a> can then perpetuate the problem – it’s not helpful to lie in bed awake willing ourselves to sleep, or to catastrophise about our sleeplessness.</p>
<p>Studies <a href="http://www.smrv-journal.com/article/S1087-0792(12)00010-X/abstract">focusing on large numbers of twin pairs</a> back up the idea that environmental influences are an important explanation for why sleep quality differs between one person and another. But they also highlight that sleep problems run in families: if you struggle with your sleep, perhaps your parents or grandparents did too. Looking at why this might be, it seems that our genes are important when it comes to our aptitude for sleeping soundly. </p>
<p>We’re learning more all the time about which specific genes might be important, as I explore in <a href="https://bloomsbury.com/uk/nodding-off-9781472946188/">Nodding Off</a>, my new book on the science of sleep. Some more recent research into this has been conducted on a vast scale. For example, one <a href="https://www.biorxiv.org/content/early/2018/02/01/214973">study of over a million people</a> identified genetic variants associated with insomnia, enriching our knowledge of the biological pathways by which insomnia develops.</p>
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Read more:
<a href="https://theconversation.com/insomnia-is-not-just-in-the-mind-27841">Insomnia is not just in the mind</a>
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<p>The complexity of the underlying causes of sleep problems goes further than this, and it’s been proposed for some time that <a href="https://www.annualreviews.org/doi/abs/10.1146/annurev-psych-010416-044053">genes and the environment go hand-in-hand</a>. For example, some people are more likely to be exposed to certain environmental experiences (such as work stresses) in part for genetic reasons. Their sensitivity to these experiences (such as whether they will keep them up at night) is also influenced by our genes. Another example of genetic and environmental interplay is <a href="https://jamanetwork.com/journals/jama/fullarticle/2441270">epigenetics</a>, which means “above genetics”. Our genes do not change, but how they influence us (whether they are “switched on or off” or “turned up or down” like a dimmer switch) can be influenced by the environment.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/221561/original/file-20180604-175425-29m9up.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/221561/original/file-20180604-175425-29m9up.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/221561/original/file-20180604-175425-29m9up.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/221561/original/file-20180604-175425-29m9up.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/221561/original/file-20180604-175425-29m9up.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/221561/original/file-20180604-175425-29m9up.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/221561/original/file-20180604-175425-29m9up.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">Do your genes cause you to sleep badly?</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-illustration/dna-molecule-spiral-structure-unique-connection-406788043?src=aX4r3gNosQXDG11umgpafQ-1-12">Konstantin Faraktinov/Shutterstock.com</a></span>
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<h2>Can it be fixed?</h2>
<p>So what does this all mean for resolving a sleep problem? You may think that the discovery that genes explain some of the differences between us in terms of our sleep quality means that some of us are destined to sleep poorly and there is nothing much we can do about it. But, thankfully, that does not follow.</p>
<p>One of the very first lessons that a student of behavioural genetics learns is that just because something is influenced by our genes does not mean that changing the environment can’t be the solution. The example so often given is that of <a href="https://www.nhs.uk/conditions/phenylketonuria/">phenylketonuria</a> (PKU). This is a disorder in which the substance phenylalanine (found in certain foods) can’t be broken down by the body and can lead to brain damage. While this is a genetic disorder, the solution lies in the environment: by carefully considering diet, the negative effects of this disorder can be prevented from developing. </p>
<p>It’s clear that cognitive behavioural therapy for insomnia (CBT-I) which addresses thoughts about sleep, as well as behaviour (including a relaxation component and making sure people do not spend time in bed awake) is the <a href="http://annals.org/aim/fullarticle/2518955">best initial treatment</a> for those who suffer from chronic insomnia. </p>
<p>But understanding more about differences between people might also eventually be useful when it comes to treatment. There is current research interest in <a href="https://theconversation.com/how-science-is-using-the-genetics-of-disease-to-make-drugs-better-30747">personalised medicine</a>, with the hope that treatment can eventually be <a href="https://onlinelibrary.wiley.com/doi/full/10.1002/da.22292">further tailored to the individual</a>. </p>
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Read more:
<a href="https://theconversation.com/personalised-medicine-how-science-is-using-the-genetics-of-disease-to-make-drugs-better-30747">Personalised medicine: how science is using the genetics of disease to make drugs better</a>
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<p>Nobody should feel that sleeplessness is something they simply have to endure. If you are struggling with sleep problems, talk to a doctor and try to reach a sleep expert. Different CBT-I <a href="https://academic.oup.com/sleep/article/39/1/13/2726046">online courses</a> are being developed and tested and some appear to be helpful. Despite an array of different causes of sleeplessness, there is help at hand for a better night.</p><img src="https://counter.theconversation.com/content/93533/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Alice M Gregory's book Nodding Off: The Science of Sleep, on which this article is based, was published by Bloomsbury in June, 2018. She has provided guidance and educational content for babysleep.com, a website partially supported by Johnson and Johnson, who do not have any influence over content and do not advertise on it. She has previously received funding to support her work from multiple sources including the MRC, ESRC, Leverhulme Trust and the British Academy. </span></em></p>Our genes are important when it comes to aptitude for sleeping soundly.Alice M Gregory, Professor of Psychology, Goldsmiths, University of LondonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/982712018-06-14T09:21:25Z2018-06-14T09:21:25ZAnthill 26: Twins<figure><img src="https://images.theconversation.com/files/223039/original/file-20180613-32347-1bze5k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Double trouble? </span> <span class="attribution"><span class="source">shutterstock.com</span></span></figcaption></figure><p>Double trouble, two peas in a pod, always together and possibly even reading each other’s minds – twins come with tonnes of stereotypes. This episode of The Anthill digs into some of the research around twins – from what it’s like to be one, how it differs to other sibling relationships (if at all) and how twins play a crucial role in scientific research.</p>
<p>As well as speaking to some twins to find out some of their pet peeves about how the world views their relationship, we speak to the University of Central Lancashire’s Kate Bacon about the extent that the identity of twins is shaped by being a twin. And psychologist Alison Pike at Sussex University gives us an overview of how twins compare to other siblings.</p>
<p>Then we explore just how much stereotypes of twins abound in popular culture. As Xavier Aldana Reyes, senior lecturer in English literature and film at Manchester Metropolitan University, told us, Gothic literature and horror films <a href="https://theconversation.com/seeing-double-the-origins-of-the-evil-twin-in-gothic-horror-and-hollywood-98196">have embraced the idea</a> of the “evil twin”. Think The Shining. </p>
<p>In films or photographs, twins are usually dressed the same and are incredibly close. But as James Hoctor, from the University of Kent’s department of philosophy told us, these depictions of twins affect how they’re viewed in the real world – and this can be damaging.</p>
<p>Aside from their use in the plot lines of scary films, twins are helping scientists understand the human condition, and particularly the impact of our genes on our lives. To find out more we spoke to three scientists who work with large scale twin databases. </p>
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<img alt="" src="https://images.theconversation.com/files/223047/original/file-20180613-32310-114dt5b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/223047/original/file-20180613-32310-114dt5b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=428&fit=crop&dpr=1 600w, https://images.theconversation.com/files/223047/original/file-20180613-32310-114dt5b.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=428&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/223047/original/file-20180613-32310-114dt5b.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=428&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/223047/original/file-20180613-32310-114dt5b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=538&fit=crop&dpr=1 754w, https://images.theconversation.com/files/223047/original/file-20180613-32310-114dt5b.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=538&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/223047/original/file-20180613-32310-114dt5b.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=538&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">Twins are helping scientists understand ageing.</span>
<span class="attribution"><span class="source">from www.shutterstock.com</span></span>
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<p>Claire Steves, a senior clinical lecturer at King’s College London, which runs the Twins UK database explained what they’ve found out comparing identical and non-identical twins. Kaare Christensen, head of the Danish Twin Registry at the University of Southern Denmark, tells us about how twins studies are revealing insights into how we age – and when we die. And Athula Sumithapala, professor of psychiatry at the University of Keele, tells us about a new twins registry he and colleagues are setting up in Sri Lanka, and why more twin databases are needed in the developing world. </p>
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<p><em>The Anthill theme music is by Alex Grey for Melody Loops. Music in the twins and popular culture segment is <a href="https://incompetech.com/music/royalty-free/index.html?isrc=USUAN1100254">Double Drift</a> by Kevin MacLeod via Incompetech. Music in the twin studies segment is Old Bossa by <a href="http://www.twinmusicom.org/">Twin Musicom</a>, via <a href="https://www.youtube.com/watch?v=ecKkW_hK8xE">YouTube</a>.</em> </p>
<p><em>Click here to listen to more episodes of The Anthill, on themes including <a href="https://theconversation.com/anthill-22-sex-91797">Sex</a>, <a href="https://theconversation.com/anthill-25-intuition-96677">Intuition</a>, and <a href="https://theconversation.com/anthill-9-when-scientists-experiment-on-themselves-71852">Scientists who experiment on themselves</a>.</em></p>
<p><em>Thank you to City, University of London’s Department of Journalism for letting us use their studios to record The Anthill.</em></p><img src="https://counter.theconversation.com/content/98271/count.gif" alt="The Conversation" width="1" height="1" />
A podcast on twins, including why stereotypes about their relationship are so damaging, and why they are so useful to scientists.Laura Hood, Senior Politics Editor, Assistant Editor, The Conversation (UK edition)Annabel Bligh, Business & Economy Editor and Podcast Producer, The Conversation UKGemma Ware, Head of AudioLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/838872017-09-26T20:06:06Z2017-09-26T20:06:06ZBad teeth? Here’s when you can and can’t blame your parents<figure><img src="https://images.theconversation.com/files/187299/original/file-20170925-17248-1bd2var.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">No, you can't blame (most) tooth decay on your parents. But for crooked teeth, the story's a little more complicated.
</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/download/confirm/407764366?size=medium_jpg">from www.shutterstock.com</a></span></figcaption></figure><p>Tooth decay, wonky teeth, yellow teeth. These are all aspects of dental health some people blame on their parents, thinking bad teeth run in the family.</p>
<p>But our <a href="http://www.sciencedirect.com/science/article/pii/S1931312817303463">recently published research</a> shows one of these – tooth decay – is largely down to environmental factors and not your genes. Just like your dentist tells you, tooth decay is almost entirely preventable.</p>
<p>So what does the evidence say about which aspects of dental health you inherit from your parents, and which you don’t?</p>
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Read more:
<a href="https://theconversation.com/four-myths-about-water-fluoridation-and-why-theyre-wrong-80669">Four myths about water fluoridation and why they're wrong</a>
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<h2>Tooth decay</h2>
<p>Sugar in the food we eat feeds communities of hundreds of different types of bacteria that live on our teeth. The acid these bacteria produce erodes the hard outer layer of our teeth (the enamel) to cause cavities (tooth decay).</p>
<p>The bacteria in our mouth that cause tooth decay aren’t present at birth. We normally acquire them shortly after, probably from other family members, and often at about the time the first teeth are starting to appear.</p>
<p>Until recently, we didn’t know much about which specific groups of bacteria were more damaging to our teeth. We also didn’t know whether someone’s genetic make-up contributed to tooth decay.</p>
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<a href="https://images.theconversation.com/files/186743/original/file-20170920-920-1o1waor.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/186743/original/file-20170920-920-1o1waor.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/186743/original/file-20170920-920-1o1waor.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/186743/original/file-20170920-920-1o1waor.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/186743/original/file-20170920-920-1o1waor.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/186743/original/file-20170920-920-1o1waor.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/186743/original/file-20170920-920-1o1waor.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/186743/original/file-20170920-920-1o1waor.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">We looked into the mouths of 485 sets of twins to see if tooth decay bacteria were inherited.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/download/confirm/608014340?src=qKObOYLcZzeQ2X0G1ItXTw-1-3&size=medium_jpg">from www.shutterstock.com</a></span>
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<p>To study the role of genes and environment in tooth decay, we worked as part of an international team to study 485 pairs of twins between the ages of five and 11. </p>
<p>These included genetically identical twins and fraternal twins, who are as genetically different as any two children in the same family.</p>
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Read more:
<a href="https://theconversation.com/seeing-double-why-twins-are-so-important-for-health-and-medical-research-5273">Seeing double: why twins are so important for health and medical research</a>
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</p>
<hr>
<p>We collected dental plaque – the sticky, bacteria-filled film – from their teeth and gums. Then our US collaborators looked at the DNA of the bacteria it contained to <a href="http://www.sciencedirect.com/science/article/pii/S0167701213002741?via%3Dihub">identify the species present</a>.</p>
<p>Although there was a genetic (inheritable) contribution to some types of tooth bacteria, these weren’t the ones associated with tooth decay.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/how-to-fill-the-gaps-in-australias-dental-health-system-35371">How to fill the gaps in Australia's dental health system</a>
</strong>
</em>
</p>
<hr>
<p>Instead, the types of bacteria that could form cavities were those influenced by environmental factors. These included negative factors (like eating sugary food) and positive factors (like brushing your teeth).</p>
<p>Bacteria associated with tooth decay were also more common in twins who consumed a lot of added sugar in their food and drinks. This association increased with age, most likely because their teeth were exposed to more sugar over time.</p>
<p><strong>Beyond bacteria</strong></p>
<p>But the story isn’t that simple. While tooth decay is largely preventable, some people are more at risk of it than others. This is why dentists sometimes look for a family history that might explain why one child might have more decay than another despite having similar diets.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/the-bitter-truth-about-what-sugar-is-doing-to-your-teeth-24340">The bitter truth about what sugar is doing to your teeth</a>
</strong>
</em>
</p>
<hr>
<p>Genes control <a href="https://doi.org/10.1017/thg.2012.78">how teeth develop</a>. And sometimes teeth <a href="https://ghr.nlm.nih.gov/condition/amelogenesis-imperfecta#">may not form properly</a>, making enamel less resistant to bacteria. Genes can also influence whether teeth will be <a href="https://link.springer.com/article/10.1007/s11599-006-9000-7">crooked</a>. All of these kinds of teeth can provide a safe harbour for bacteria because they’re difficult to clean.</p>
<p>While, generally, you can’t blame your parents for decay, in these specific cases, you can. Either way, you still need to brush your teeth and avoid sugary food and drinks, just as your dentist tells you. </p>
<h2>Crooked teeth</h2>
<p>The type of tooth misalignment treated by braces also seems to be influenced by a combination of <a href="https://link.springer.com/article/10.1007/s11599-006-9000-7">genes and environment</a>.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/186742/original/file-20170920-920-1g8l3uy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/186742/original/file-20170920-920-1g8l3uy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/186742/original/file-20170920-920-1g8l3uy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/186742/original/file-20170920-920-1g8l3uy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/186742/original/file-20170920-920-1g8l3uy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/186742/original/file-20170920-920-1g8l3uy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/186742/original/file-20170920-920-1g8l3uy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/186742/original/file-20170920-920-1g8l3uy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The type of tooth misalignment treated by braces also seems to be influenced by a combination of genes and environment.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/download/success?src=lc3rt2c29gBQpKurKAAq6w-1-22">from www.shutterstock.com</a></span>
</figcaption>
</figure>
<p>The way teeth are arranged depends on a whole range of factors, including the number and size of the teeth, the size of the jaw, how the teeth come together when we chew, and the balancing forces between the tongue and lips. Many of these have a genetic origin.</p>
<p>But certain behaviours (which themselves can also have a genetic component) may also play a role in whether your teeth are crooked. These include using a dummy, <a href="https://www.merriam-webster.com/medical/tongue%20thrust">thrusting your tongue</a> forward against your teeth, and sucking your thumb. If environmental factors didn’t influence how our teeth are aligned, orthodontics (like braces) wouldn’t work.</p>
<p>So, for crooked teeth, you can (partly) blame your parents.</p>
<h2>Yellow teeth</h2>
<p>Tooth colour is determined, once again, by a combination of our <a href="http://onlinelibrary.wiley.com/doi/10.1111/j.1708-8240.1999.tb00413.x/full">genes and our environment</a>. The way in which the white enamel (and the underlying yellow dentine) forms during development is mainly due to our genes. People whose teeth develop naturally with thinner enamel generally have teeth that appear more yellow, although this can also be a consequence of a loss of enamel as we age. </p>
<p>Environmental factors that play a role in tooth colour include intrinsic factors (things incorporated into the tooth while it develops). These include being exposed to the antibiotic tetracycline in the womb, or to excess fluoride as a child.</p>
<p>Then there are extrinsic factors (things incorporated into the tooth after it has developed) that affect tooth colour. These include drinking coffee or smoking. </p>
<p>So, you can only partly blame your yellow teeth on your parents.</p>
<h2>When teeth appear</h2>
<p>Twin studies are also <a href="https://www.adelaide.edu.au/press/titles/twin-studies/twin-studies-ebook.pdf">telling us more</a> about how genes influence early life events. For instance, the age at which our <a href="http://dx.doi.org/10.1375/twin.13.6.573">first baby teeth emerge</a> and the <a href="http://dx.doi.org/10.1111/j.1834-7819.2010.01230.x">sequence in which they emerge</a> seem to be strongly controlled by our genes.</p>
<p>So, people whose baby teeth emerged early can blame their parents.</p>
<h2>In a nutshell</h2>
<p>While some aspects of bad teeth are mainly controlled by genes, others are largely down to environmental factors, and others are controlled by both. And studying twins is beginning to tell us which is which.</p>
<p>But whichever way the evidence lands, your dentist’s message will be the same. Avoid sugary food and drink, brush your teeth and have regular check-ups.</p><img src="https://counter.theconversation.com/content/83887/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jeffrey Craig receives funding from the National Institute of Dental and Craniofacial Research of the National Institutes of Health (Award Number R01DE019665); the National Health and Medical Research Council (grant number 1079102), the Financial Markets Foundation for Children (grant number 2015-111), the Cerebral Palsy Alliance Research Foundation (grant number PG2316), and from the Victorian Government’s Operational Infrastructure Support Program. He is affiliated with the Early Life Nutrition Coalition the Australia and New Zealand Society for the Developmental Origins of Health and Disease, the International Society of twin Studies, and the Prader Willi Research Foundation Australia. </span></em></p><p class="fine-print"><em><span>Pamela Leong receives funding from the National Institute of Dental and Craniofacial Research of the National Institutes of Health (Award Number R01DE019665)</span></em></p><p class="fine-print"><em><span>Toby Hughes receives funding from the National Institutes of Health; the National Health and Medical Research Council; The Australian Dental Research Foundation; The Financial Markets Foundation for Children; The Channel 7 Children's Research Foundation. He is affiliated with the International Association for Dental Research and the International Society of Twin Studies.</span></em></p>Can you blame bad teeth on your genes? Here’s why the answer is not as simple as you might think.Jeffrey Craig, Principal Research Fellow, Murdoch Children's Research InstitutePamela Leong, Study coordinator Peri/Postnatal Epigenetics Twin Study, Murdoch Children's Research InstituteToby Hughes, Associate Professor, University of AdelaideLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/812912017-08-01T08:58:38Z2017-08-01T08:58:38ZSocial mobility is in your genes, finds new study<figure><img src="https://images.theconversation.com/files/179852/original/file-20170726-2133-1sj5mrg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Going up. </span> <span class="attribution"><span class="source">via www.shutterstock.com</span></span></figcaption></figure><p>For decades, researchers <a href="https://www.sociologicalscience.com/download/volume-2/february/SocSci_v2_82to105.pdf">have linked</a> a person’s social mobility to their education. The more educated you are the more likely you are to be upwardly socially mobile. And if your parents attended university, you are more likely to do so too. </p>
<p>This link between the education level of parents and their children has been considered to be caused purely by the environment in which children were brought up. But as <a href="http://journals.sagepub.com/doi/pdf/10.1177/0956797617707270">our new study</a> shows, genes – passed down from parents to their children – do play a role. In fact, we found that genes explained half of differences in whether offspring are socially mobile or not.</p>
<p>We tested for genetic influence on social mobility in more than 6,000 families from the <a href="https://www.teds.ac.uk/">Twins Early Development Study</a>, which is funded by the Medical Research Council. As the picture below shows, we measured genetic influence on four categories of social mobility: stably educated, stably uneducated, upwardly mobile and downwardly mobile. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/179422/original/file-20170724-19173-1w9mb30.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/179422/original/file-20170724-19173-1w9mb30.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/179422/original/file-20170724-19173-1w9mb30.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=367&fit=crop&dpr=1 600w, https://images.theconversation.com/files/179422/original/file-20170724-19173-1w9mb30.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=367&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/179422/original/file-20170724-19173-1w9mb30.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=367&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/179422/original/file-20170724-19173-1w9mb30.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=462&fit=crop&dpr=1 754w, https://images.theconversation.com/files/179422/original/file-20170724-19173-1w9mb30.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=462&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/179422/original/file-20170724-19173-1w9mb30.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=462&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The four social mobility groups.</span>
<span class="attribution"><a class="source" href="https://www.youtube.com/watch?v=ZTrbsjAvRaA">King's College London</a></span>
</figcaption>
</figure>
<p>We used two methods to test for genetic influence on social mobility: one measured the contribution of genetic factors by studying large samples of twins, while the other tested for actual DNA differences between unrelated people within the study. </p>
<h2>Like with unlike</h2>
<p>In the first method – the standard <a href="https://oup.silverchair-cdn.com/oup/backfile/Content_public/Journal/bib/3/2/10.1093/bib/3.2.119/2/119.pdf?Expires=1501061538&Signature=d2fi%7EL7aXCOg3BUx521T3C4ML7O5POcxedzaHQlTxVktoTFukE-6Hh-pKcFgcoYR2A15xJ-axhQV88TJHe4%7ELm%7Ebia%7EWb2DEKTpDXv7zTTHJN7pQruvUns0ynLSVbD0-mujc7qG1UwmJgf06ijqg1N9DWsh5kbnoGZfAjPL2s9XC6rY2LdWRewVZ%7ELycICV7JKQvwesR1FDCpUFm7k6tdQOVTVTuQpkhQ5TNI5zAzSYn6j6aTJw-2SpPriZwM1Z4LLm-z14HVm3rlXKQIMn9qpXJx-ZlfhLjeDC1bCJCEf9Jzx2Hlyc79P7PDYR8du4duLaHl9UJwo6PyUQRrhMRCg__&Key-Pair-Id=APKAIUCZBIA4LVPAVW3Q">twin design</a> test – we compared rates of social mobility between identical twins who share 100% of their genetic material, and non-identical fraternal twins, who share 50% of the genes that differ between people. If identical twins are more similar than non-identical twins on social mobility this indicates genetics is playing a role. </p>
<p>For example, we found that if one twin in an identical pair is upwardly mobile, the likelihood that their twin was also upwardly mobile was greater than if they were non-identical twins. </p>
<p>Overall, we found that almost 50% of differences in whether families were socially mobile or not are due to their genes. This means that across the population, inherited DNA differences contribute substantially to how socially mobile people in the UK are. This statistic cannot, however, be applied to an individual – so this does not mean that whether any given child is socially mobile or not is 50% due to their genes. This is because estimates from twin studies tell us about differences between individuals in a particular population at a particular time.</p>
<h2>Marks on your DNA</h2>
<p>The second method we used was called “polygenic scoring”. To create a polygenic score we inspected the genome of 5,825 unrelated members of our study sample (one randomly selected twin from a pair). We looked at thousands of genetic variants that have previously been linked to <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4883595/pdf/nihms770340.pdf">educational attainment</a> and added up the number of education-associated bits of DNA that each person has to create what we call a <a href="http://journals.plos.org/plosgenetics/article/file?id=10.1371/journal.pgen.1003348&type=printable">polygenic score</a>. </p>
<p>We then compared the polygenic scores of children in each of our four categories of social mobility to see whether they differed in the number of education-related genetic variants they carried in their genome. We found that people who enrolled in higher education had higher polygenic scores – so more education-associated bits of DNA – even if they had come from families where neither parent had gone to university. Because polygenic scores are based on an individuals’ DNA, these results can be applied to an individual person. Our polygenic score findings indicate that a child’s genetic propensity for education contributes to whether they will be socially mobile or not, irrespective of the social status of the family they were born into.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/ZTrbsjAvRaA?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
</figure>
<h2>Reducing educational inequality</h2>
<p>Our results indicate that both similarities and differences in educational attainment between parents and their offspring are influenced by genetic factors. As parents and their children are not genetically identical, our results show that children with a high genetic propensity for educational success who are born to parents without a university degree can surpass these constraints and reach higher education – in part due to their genes.</p>
<p>Knowing to what extent genetic differences contribute to social mobility can tell us how equal educational opportunities are in a society at a given time. This is because as environmental opportunities broaden in a society, for example as every child now has to attend school, the influence of genes becomes more apparent. Centuries ago, only the very rich were able to attend school. This made their genes, or inherent ability, less relevant to their success, and their environment – in this case wealth – more important.</p>
<p>In other words, if inequalities are reduced in a society, people’s differences – in educational attainment and elsewhere – will increasingly become a function of their genetic differences and less of inequalities in their environments. These results tell us that if we want to reduce educational inequalities, it’s important to understand childrens’ genetic propensity for educational achievement. They also suggest that a dialogue about improving social mobility needs to include genetics, as it is an important factor which contributes to educational outcomes.</p><img src="https://counter.theconversation.com/content/81291/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>The authors do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Research finds genes account for 50% of differences in social mobility.Ziada Ayorech, PhD Candidate, Social Genetic and Developmental Psychiatry Centre, King's College LondonSophie von Stumm, Senior Lecturer in Psychology, Goldsmiths, University of LondonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/796772017-06-20T20:33:57Z2017-06-20T20:33:57ZHow 50 medical experts separated Kenyan conjoined twins in 23-hour surgery<figure><img src="https://images.theconversation.com/files/174881/original/file-20170621-30165-1qvzij2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Former conjoined twins, Blessing and Favour, after a successful surgery at Kenyatta National Hospital. </span> <span class="attribution"><span class="source">Kenyatta National Hospital</span></span></figcaption></figure><p><em>The successful <a href="http://www.nation.co.ke/news/conjoined-twins-are-separated/1056-3439186-9150ik/index.html">separation</a> of two year old conjoined twins at Kenyatta National Hospital marked a medical milestone in Kenya. Other than South Africa, <a href="https://www.ncbi.nlm.nih.gov/pubmed/16122113">very few successful</a> separations have been performed in <a href="http://www.bioline.org.br/pdf?rh09041"> sub-Saharan Africa</a>. In Kenya a multi-disciplinary team of medical experts operated on the twins for 23 uninterrupted hours. The Conversation Africa’s health and medicine editor Joy Wanja Muraya spoke to Dr Joseph Wanjeri about the surgery and post recovery of the twins who are home.</em></p>
<p><strong>Can you explain conjoined twins, and its prevalence?</strong></p>
<p><a href="http://www.mayoclinic.org/diseases-conditions/conjoined-twins/home/ovc-20198079">Conjoined twins</a> are two babies born physically connected to each other. The extent and site of their union varies from sharing a band of skin and underlying tissues to more complex varieties <a href="http://www.umm.edu/programs/conjoined-twins/facts-about-the-twins">sharing</a> vital organs.</p>
<p>Research shows that cases of conjoined twins are found in one of every <a href="https://www.innovativepublication.com/admin/uploaded_files/IJOGR_3(2)_181-184.pdf">50,000</a> <a href="https://www.ncbi.nlm.nih.gov/pubmed/16122113">live births</a> globally. Kenya does not have a central repository for such cases.</p>
<p>Conjoined twins are the result of the embryo cells that have not completely <a href="https://sonoworld.com/fetus/page.aspx?i=2017">separated</a>. Embryo cells develop when the egg (ovum) is fertilised by sperm. They multiply and differentiate to form different body organs and tissues. An alternative <a href="http://example.com/">theory</a> is that two separate embryos fuse together in the early development of the twins. </p>
<p>The exact cause of conjoined twins is unknown but it’s thought that genetic factors interacting with environmental ones may contribute. Another possibility is the medicines taken by the mother <a href="http://emedicine.medscape.com/article/934680-overview#a8">during pregnancy</a>. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/174427/original/file-20170619-12397-cf9a2p.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/174427/original/file-20170619-12397-cf9a2p.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/174427/original/file-20170619-12397-cf9a2p.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/174427/original/file-20170619-12397-cf9a2p.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/174427/original/file-20170619-12397-cf9a2p.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/174427/original/file-20170619-12397-cf9a2p.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/174427/original/file-20170619-12397-cf9a2p.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The twins when they were first admitted at Kenyatta National Hospital on 5th September 2014. Photo: Author Provided.</span>
</figcaption>
</figure>
<p>Conjoined twins are classified based on the place they are joined. The most common types of conjoined twins are:</p>
<ul>
<li><p><a href="https://academic.oup.com/brain/article-abstract/129/5/1084/327141/The-craniopagus-malformation-classification-and?redirectedFrom=PDF">craniopagus</a> – fusion of the head</p></li>
<li><p><a href="http://emedicine.medscape.com/article/934680-overview">thoracopagus</a> – fusion of chest</p></li>
<li><p><a href="https://pediatriceducation.org/2012/04/30/what-are-the-different-types-of-conjoined-twins/">Omphalopagus Parapagus</a> – lateral fusion </p></li>
<li><p>Ischiopagus and <a href="http://www.mayoclinic.org/diseases-conditions/conjoined-twins/symptoms-causes/dxc-20198093">Sacropagus</a> – fusion at the lower back or sacrum</p></li>
</ul>
<p>Seperating conjoined twins can be difficult and can result in death. Complex cases can be inoperable and others may call for emergency surgery soon after birth if the life of the twins is threatened. </p>
<p>Separating twins that are conjoined at the lower back (sacropagus) has fewer complications and deaths. The Kenyan twins were conjoined in this way.</p>
<p>Expertise, careful preparation and team work is the perfect recipe for a successful outcome of the surgery.</p>
<p><strong>How is diagnosis done?</strong></p>
<p><a href="http://www.mayoclinic.org/diseases-conditions/conjoined-twins/diagnosis-treatment/diagnosis/dxc-20198110">Diagnosis</a> can be done before birth using an ultrasound scan or through physical examination at birth. Advanced tests like the Computerised Tomography (CT) scans and Magnetic Resonance Imaging (MRI) give greater details. </p>
<p>In the case of the Kenyan twins, a set of female conjoined twins was referred to the Kenyatta National Hospital in Nairobi in September 2014 from an upcountry health facility soon after their delivery. </p>
<p>The twins shared a spinal cord, rectum, anus, some muscles, subcutaneous tissues and skin. Paediatric, neuro and plastic surgeons agreed that separation was feasible but it should wait until the twins were bigger to withstand the complex surgery. </p>
<p>Paediatricians, nurses and nutritionists took care of the twins until they were one year old when the planning for the separation began. The plastic surgery team recommended tissue expansion, a procedure to help with closure of the huge soft tissue defects anticipated after separation. But the mother refused to give her consent, and the procedure had to be stopped. She clearly adored the twins but was overwhelmed by the decisions that needed to be taken. The matter was referred to court and the judge ruled that separation was in the best interests of the children. Their mother gave consent and the preparations for surgery were resumed.</p>
<p>A University of Nairobi plastic surgery resident created a 3D model of the twin’s pelvis to map the surgery.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/174424/original/file-20170619-12416-1hags0s.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/174424/original/file-20170619-12416-1hags0s.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/174424/original/file-20170619-12416-1hags0s.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/174424/original/file-20170619-12416-1hags0s.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/174424/original/file-20170619-12416-1hags0s.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=502&fit=crop&dpr=1 754w, https://images.theconversation.com/files/174424/original/file-20170619-12416-1hags0s.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=502&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/174424/original/file-20170619-12416-1hags0s.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=502&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A three-dimensional (3D) impression of the twins conjoinment. Kenyatta National Hospital.</span>
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</figure>
<p><strong>Can you explain the details of the planning and actual surgery.</strong></p>
<p>A multi-disciplinary medical team made up of 50 experts drawn from various medical fields did a dry run a week before the actual operation. </p>
<p>When the time came for the operation in November 2016 two sets of the anaesthetic teams took about three hours to anaesthetise and stabilise the twins.</p>
<p>The paediatric surgeons began the separation of the various soft tissues on one side up to the spine. The neurosurgeons took over and split the spine, opened the shared <a href="http://www.medicinenet.com/script/main/art.asp?articlekey=32512">dura</a>,which is the tough outermost membrane enveloping the brain and spinal cord. They later separated the nerves before repairing the dura.</p>
<p>The paediatric surgeons completed their separation of the soft tissue and successfully placed the pair on separate beds for the first time. But one was left without an anus and rectum. This condition would be repaired in the recovery phase.</p>
<p>The closure of the huge soft tissue defects and wounds was done on each of the girls by plastic surgeons. Local flaps and skin grafts covered the wounds after which an opening from the large intestine – a colostomy – was done.</p>
<p>They were transferred to the intensive care unit for specialised monitoring for two weeks before transfer to the paediatric surgical ward.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/174442/original/file-20170619-12416-1xs71t5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/174442/original/file-20170619-12416-1xs71t5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/174442/original/file-20170619-12416-1xs71t5.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/174442/original/file-20170619-12416-1xs71t5.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/174442/original/file-20170619-12416-1xs71t5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/174442/original/file-20170619-12416-1xs71t5.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/174442/original/file-20170619-12416-1xs71t5.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Blessing and Favour face each other for the first time since birth. Photo: Kenyatta National Hospital.</span>
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</figure>
<p>The wounds took about three months to heal. One of the twins developed impaired movement of one of her ankle joints. But after occupational therapy she was able to walk with only a slight limp.</p>
<p>The twin’s growth and development will be monitored in the coming months and further reconstructive surgeries scheduled at the appropriate time.</p>
<p>Blessing and Favour were <a href="http://www.nation.co.ke/news/separated-twins-finally-leave-hospital/1056-3972796-t3ju92/index.html">discharged</a> from hospital on June 15, 2017.</p>
<p><strong>What does this success mean for sub-Saharan Africa and what lessons can be learnt?</strong></p>
<p>First, it shows that medical experts in sub-Saharan Africa are qualified and capable of performing complex surgeries in spite of the poor resources at their disposal. </p>
<p>But it also shows that a referral centre with a multidisciplinary team is best suited for this kind of complex surgery.</p>
<p>And as far as the children are concerned, it shows that intense nursing care, nutritional support, advanced wound care techniques and close monitoring by paediatric specialists has been critical to their remarkable recovery.</p><img src="https://counter.theconversation.com/content/79677/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Joseph Kimani Wanjeri does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Successfully separating conjoined twins is a complex operation that depends on how they are joined as well as the experience and skill of the surgical team.Joseph Kimani Wanjeri, Lecturer - Dept. of Surgery, Speciality: Plastic & Reconstructive Surgery , University of NairobiLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/744962017-03-15T10:07:41Z2017-03-15T10:07:41ZGut bacteria play a role in long-term weight gain<figure><img src="https://images.theconversation.com/files/160697/original/image-20170314-10720-1g3bqgp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/download/confirm/531323893?size=medium_jpg">Sharomka/Shutterstock.com</a></span></figcaption></figure><p>Weight gain happens when we consume more food than we can burn, and weight loss happens when we burn more energy than we consume. But why do some people seem to eat whatever they want and not gain weight, and others appear to gain weight even if they eat reasonable amounts of food? The answer, at least in part, may be found in the bacteria that live in our guts. </p>
<p>Our latest research, published in the <a href="http://www.nature.com/ijo/journal/vaop/naam/abs/ijo201766a.html">International Journal of Obesity</a>, shows that people who have a stable weight over nine years or lose weight, have a larger number of different types of microbes in their guts, eat more fibre and have a higher abundance of certain types of gut microbes.</p>
<p>In the past decade, researchers have found that the microbes in our gut have a strong effect on various aspects of our health. Studies in mice have demonstrated that how the body converts food into energy depends in large part on the different types of microbes a person has in their gut and also on the kind of microbes they carry.</p>
<p>In a <a href="http://theconversation.com/why-frequent-dieting-makes-you-put-on-weight-and-what-to-do-about-it-69329">recent study</a>, scientists in Israel found that mice who were put on a yo-yo diet slowly gained weight compared with mice on a steady diet – despite the fact that both groups received the same amount of calories overall. </p>
<p>One of the effects seen in the mice that were put on the yo-yo diet was a decrease in their gut microbiome diversity. Also, when they transplanted the microbes from the yo-yo dieters into the guts of non-yo-yo dieters, the mice on steady diets gained weight – showing that the altered microbes were the cause of the weight gain. But is this relevant to humans?</p>
<p>In humans, comparing microbes in the gut in obese and thin individuals, scientists have already shown that lean people have many more species of intestinal bacteria than obese people.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/160739/original/image-20170314-10720-1e5u042.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/160739/original/image-20170314-10720-1e5u042.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/160739/original/image-20170314-10720-1e5u042.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/160739/original/image-20170314-10720-1e5u042.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/160739/original/image-20170314-10720-1e5u042.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/160739/original/image-20170314-10720-1e5u042.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/160739/original/image-20170314-10720-1e5u042.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">I’ve been on a yo-yo diet.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/download/confirm/406573012?src=230e2_hfgEaJo1Sf4S3W7A-1-2&size=medium_jpg">Janson George/Shutterstock.com</a></span>
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</figure>
<h2>What twins taught us</h2>
<p>Until now, however, there were no experiments tying the gut microbes to changes in weight over several years. For this reason, we decided to do an <a href="http://www.nature.com/ijo/journal/vaop/naam/abs/ijo201766a.html">investigation</a> into 1,632 women from the UK, all of them twins (about half of them identical). The participants had their body weight measured several years ago and, back then, they answered questions about the amounts and types of foods that they ate. We called them again nine years later and, in addition to measuring their weight, we asked them to give us a poo sample so we could analyse the bacteria in their gut.</p>
<p>We found that most of the women gained weight over the nine years, but this was not fully explained by the number of calories in their diet when the study began. Because they are twins, it was also possible to calculate (using the differences between identical and non-identical twins) how much of the weight gain can be explained by genes. Only 41% of the change in weight was explained by genes. That meant that there were other factors, in addition to genes and calories. </p>
<p>We discovered that women who ate high amounts of dietary fibre (found in fruit, vegetables and whole grains) were less likely to gain weight than those who ate little fibre, even if they consumed roughly the same amount of calories. Women who lost weight or had stable weight also had more diverse microbes in their guts. We were able to pinpoint some of the microbes that are different between women who had gained weight and those who had lost weight. Most of these microbes had already been discovered in mice to be involved in better energy metabolism.</p>
<p>These results show that the exciting studies in mice about how microbes affect weight gain are also relevant in humans. They are also important because they will allow our group, and other scientists, to investigate how to influence a person’s gut microbes – using probiotics and fibre – so they are at a lower risk of developing obesity.</p><img src="https://counter.theconversation.com/content/74496/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Ana Valdes does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>The make-up of your gut bacteria will determine whether or not you put on weight.Ana Valdes, Associate Professor and Reader, University of NottinghamLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/734162017-02-23T11:23:33Z2017-02-23T11:23:33ZHow genetics can uncover links in chronic pain and other conditions<figure><img src="https://images.theconversation.com/files/157877/original/image-20170222-1316-zf19do.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Chronic pain can be disabling.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/backache-young-afroamerican-man-shirt-tie-223835530?src=ImckOuTyC1wrizxWO8l-9w-2-3">Shutterstock</a></span></figcaption></figure><p>In the recent <a href="http://thelancet.com/journals/lancet/article/PIIS0140-6736(15)60692-4/fulltext">Global Burden of Disease</a> study, four of the top ten causes of disability worldwide were chronic pain conditions. Chronic pain is defined as pain that lasts beyond normal healing time – usually three months – and is one of the most common global causes of incapacity. It rarely occurs by itself, however, and is one of the <a href="http://www.thelancet.com/journals/lancet/article/PIIS0140-6736(12)60240-2/abstract">most common conditions to present itself alongside other chronic conditions</a>, such as diabetes and <a href="https://www.blf.org.uk/support-for-you/copd">COPD</a>. This increases the overall burden of disability, and the impact of each chronic condition. </p>
<p>The exact reason why some people suffer from several chronic diseases and others don’t, is not well understood. However, <a href="http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0170653">we have discovered</a> that genetics could partially explain this. </p>
<p>Two of the most common disorders which occur alongside chronic pain are depression and angina. There is already <a href="http://www.thelancet.com/journals/lancet/article/PIIS0140-6736(12)60240-2/abstract">evidence of shared socio-demographic risk factors</a> for all of these conditions, particularly older age and social deprivation, as well as lifestyle factors. However these do not explain all of the shared risk.</p>
<figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/157894/original/image-20170222-1344-cn1ljg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/157894/original/image-20170222-1344-cn1ljg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/157894/original/image-20170222-1344-cn1ljg.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/157894/original/image-20170222-1344-cn1ljg.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/157894/original/image-20170222-1344-cn1ljg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=502&fit=crop&dpr=1 754w, https://images.theconversation.com/files/157894/original/image-20170222-1344-cn1ljg.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=502&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/157894/original/image-20170222-1344-cn1ljg.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=502&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Chronic pain can often bring other issues with it, like depression.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/88786104@N08/8674053127/in/photolist-JjtjnJ-gTTyUH-gTRXiL-gTSdGn-eduMN8-gTScLJ-5LxUE-FE7JK-nhRkoY-8TZBhu-8jS8Ha-gTRXRS-AjQ3u-iNzFk-8eqcL6-7W5wfz-cVppZU-Qp2K-9bCsqs-9ZycEo-5uWyBF-4P5vg1-BVeth3-jsZp4A-4ai2Sq-fonJi5-5cVPQG-4xbsmp-cVqwsb-6bPTwP-gPST7H-2EAepd-ankD3Q-QM734g-HdEYX-biT4qv-aAMm7n-ogYUKT-pcL8eC-7ir23q-a3xacY-dFGJXK-j9HbdW-67c7JC-ruvs6n-q6Sz2A-mZft3a-ofGjiD-cVprbU-2fCJPR">mattwalker69/flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>In order to investigate a risk within families and a genetic explanation for chronic disease, we examined two major groups, for the co-occurrence of chronic pain, depression and heart disease in individuals and their siblings. </p>
<h2>Family links</h2>
<p>Data from <a href="http://www.ed.ac.uk/generation-scotland/about">Generation Scotland</a> included 24,000 individuals, recruited in family groups, with data on multiple chronic illnesses, socio-demographic and psychological factors, and blood from which DNA was genetically analysed. When the data was collected, 18% of participants reported chronic pain, 13% had a history of major depressive disorder and 10% had angina. </p>
<p>We looked at the existence of two or three of these conditions in individuals and we found that people with depression were two and a half times more likely to experience chronic pain; while people with both depression and heart disease were nine times more likely to experience chronic pain. It is clear that the existence of one condition increases the chance of an individual having another, or both of the other conditions.</p>
<p>A familial risk was confirmed when we looked at siblings of people affected by these conditions. A sibling of someone with heart disease was twice as likely to have chronic pain, and siblings of those with depression were twice as likely to suffer from heart disease. This suggests that genetics plays a part in these chronic diseases, in addition to known social and demographic factors. </p>
<p>The magnitude of a shared genetic explanation for these chronic conditions was examined by looking at sets of twins. <a href="http://www.twinsuk.ac.uk/">TwinsUK</a> has data on 12,000 identical and non-identical twins from across the UK, of 16-98 years of age. In a sample of 2,902 of these, 20% suffered with chronic pain, 22% had depression and 35% reported a cardiovascular disease. </p>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/157883/original/image-20170222-1319-11oflva.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/157883/original/image-20170222-1319-11oflva.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=900&fit=crop&dpr=1 600w, https://images.theconversation.com/files/157883/original/image-20170222-1319-11oflva.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=900&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/157883/original/image-20170222-1319-11oflva.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=900&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/157883/original/image-20170222-1319-11oflva.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1130&fit=crop&dpr=1 754w, https://images.theconversation.com/files/157883/original/image-20170222-1319-11oflva.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1130&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/157883/original/image-20170222-1319-11oflva.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1130&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The health of twins can be revealing for genetics.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/studio-portrait-young-happy-twin-sisters-555764521?src=eqgeWy8WO0KO6-q-ILq8ow-3-46">Shutterstock</a></span>
</figcaption>
</figure>
<p>We compared the rates of occurrence of a condition, and of co-occurrence with another, between identical and non-identical twins. In identical twins, it was consistently more likely that both individuals would be affected, by any of the conditions, than non-identical twins, which further confirms that there is significant genetic contribution. When we examined the co-occurrence of chronic widespread pain and heart disease in our twins we found that the model that best explained the co-occurrence was a combination of both genetic and non-shared environmental factors.</p>
<h2>Treatment research could follow</h2>
<p>Although there are numerous causes of chronic pain, there are similarities in the socio-demographic factors explaining their development. Recent research shows that there are also similar biological factors present in the development of different types of chronic pain. </p>
<p>For the sufferer, it is the pain itself, rather than the cause, that produces the most distress and disability – <a href="http://onlinelibrary.wiley.com/doi/10.1016/j.ejpain.2005.06.009/abstract">most chronic pain sufferers</a> had it for more than five years at more than one site. The most common chronic pain, back pain, accounted for <a href="http://thelancet.com/journals/lancet/article/PIIS0140-6736(15)60692-4/fulltext">146m years lived with disability in 2013</a>, three times the level of depression. </p>
<p>Overall, <a href="http://onlinelibrary.wiley.com/doi/10.1016/j.ejpain.2005.06.009/abstract">19%</a> of adults in Europe, and <a href="https://www.ncbi.nlm.nih.gov/pubmed/11356737">6%</a> in the UK, were found to have significant chronic pain that was intense, severely disabling and limiting. This is similar to the prevalence of conditions such as cancer, heart disease and diabetes. </p>
<p>As well as the issue that chronic pain represents for individuals, its management places an important burden on healthcare services – and it impacts on families, society and the economy. Therefore, the finding that a genetic mechanism could help to explain the co-occurence of these conditions is significant to allow further research. The exact genes involved in the occurence and co-occurrence of chronic pain need to be identified, so that we may switch them off at an early stage and try to develop new treatments. </p>
<p>Of course, it will always be important to understand and address the socio-demographic causes of disability and co-occurrence of conditions – especially with regards to factors we could change, such as deprivation. However, our research also suggests a new model of chronic disease, based on genetics and biological factors.</p>
<p>Genes are important in determining the risk, both of chronic disease itself, and of co-occurrence of other disabilities. Only a deeper understanding of these factors will allow the development of new preventive and targeted treatments.</p><img src="https://counter.theconversation.com/content/73416/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Blair H. Smith receives research funding from the EU (Horizon 2020) and the Scottish Government, and has previously received research funding from the Medical Research Council. He is a member of the Scottish National Party.</span></em></p>Chronic pain often comes with other illnesses. Researchers have now shown that genetics can play a part in how likely you are to suffer.Blair H. Smith, Head of Population Health Sciences Division, Professor of Population Science, University of DundeeLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/675022016-11-21T14:04:39Z2016-11-21T14:04:39ZWhy it is useful to understand the role of genetics in behaviour<figure><img src="https://images.theconversation.com/files/146377/original/image-20161117-18128-1nxz4xz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption"></span> <span class="attribution"><a class="source" href="http://www.shutterstock.com/pic-373016134/stock-photo-dna-strand.html?src=4buapHyjTsYmqIZxAu9a1g-1-15">adike/Shutterstock.com</a></span></figcaption></figure><p>Scientists have studied twins for many years to understand how genes and environments influence differences among individuals, spanning conditions such as <a href="http://jamanetwork.com/journals/jama/fullarticle/2480486">cancer</a> and <a href="http://jamanetwork.com/journals/jamapsychiatry/fullarticle/208134">mental health</a> to characteristics such as <a href="https://www.ncbi.nlm.nih.gov/pubmed/11912040">intelligence</a> and <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4038932/">political beliefs</a>. </p>
<p>Although the twin method is well-established, findings from twin studies are often controversial. Critics of twin research question the value of establishing that characteristics, such as health behaviours, have a strong genetic basis. A primary concern is that these types of findings will result in complacency or fatalism, effectively undermining motivation to change lifestyle. But there is very little evidence to support these fears. </p>
<p>Genetic influence on human characteristics is often misinterpreted. It is wrongly assumed that a behaviour that has strong genetic influence (highly heritable) must be biologically hardwired. However, genes are not destiny. Genes are often dependent on environmental exposure, such that genes can have a stronger effect, or no effect, depending on the environment. </p>
<p>For example, people with a genetic predisposition to lung cancer are unlikely to develop the disease unless they smoke. The same is true of behaviour. Behaviour is only elicited in response to environmental cues. Establishing that a behaviour has an important genetic basis does not imply that this behaviour cannot be changed through environmental means.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/146374/original/image-20161117-18145-9ivbks.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/146374/original/image-20161117-18145-9ivbks.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=413&fit=crop&dpr=1 600w, https://images.theconversation.com/files/146374/original/image-20161117-18145-9ivbks.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=413&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/146374/original/image-20161117-18145-9ivbks.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=413&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/146374/original/image-20161117-18145-9ivbks.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=520&fit=crop&dpr=1 754w, https://images.theconversation.com/files/146374/original/image-20161117-18145-9ivbks.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=520&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/146374/original/image-20161117-18145-9ivbks.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=520&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Twins help us understand how genes and environments influence differences between people.</span>
<span class="attribution"><a class="source" href="http://www.shutterstock.com/pic-360752891/stock-photo-twin-girls-are-exercising-on-a-lake-shore-sprinkling-water.html?src=6-8hoVmUf2UXTCUHJ3QzjQ-1-3">JGA/Shutterstock.com</a></span>
</figcaption>
</figure>
<h2>What’s the benefit?</h2>
<p>Twin studies provide important insights into when and how genes and environments shape human nature. Studies following twins over many years have shown that the importance of genes can change dramatically <a href="https://www.ncbi.nlm.nih.gov/pubmed/21757369">with development</a> . Genetic influence tends to increase with age for many characteristics – for example, <a href="https://www.ncbi.nlm.nih.gov/pubmed/23980914">body weight</a> and <a href="https://www.ncbi.nlm.nih.gov/pubmed/23919982">intelligence</a>. It is thought that with increasing maturity comes the ability to make independent choices in line with our genetic predispositions. For example, a child who is genetically predisposed to be good at reading might join a library to gain access to more books and meet like-minded people once he or she is a teenager. Twins can therefore identify the windows of opportunity when environmental influences might be strongest, and when behaviours may be easier to change.</p>
<p>Twin studies also inform researchers where best to target environmental interventions. Interventions targeting characteristics influenced by shared environments might best be directed at the family environment. But policymakers may have greater success if interventions are oriented towards the wider environment for characteristics shaped by factors unique to each person. </p>
<p>On a broader level, twin studies are also the first step towards molecular genetic research identifying specific genes involved. One classic example is body weight. We have known from twin studies that weight has a strong <a href="https://www.ncbi.nlm.nih.gov/pubmed/17434869">genetic basis</a>, which led researchers to identify approximately 100 genetic variants involved. </p>
<p>The most important of these is FTO (the fat mass and obesity gene); and adults carrying two copies of the risk variant are heavier and at increased risk for obesity. The discovery of FTO and other variants paved the way for researchers to study the mechanisms through which genes influence weight in order to develop new drugs, and to help people with obesity to understand their vulnerability better. </p>
<h2>What are the risks?</h2>
<p>Undeniably <a href="https://theconversation.com/when-it-comes-to-food-its-not-helpful-to-believe-in-the-genetics-of-behaviour-67181">there are concerns</a> that promoting the knowledge that healthy behaviour is partially down to genes may somehow stop people from taking responsibility for managing their own, or their child’s behaviour. However, studies exploring individual feedback on DNA-based disease risk suggest that knowing your genetic predisposition does not necessarily undermine attempts to improve health, but may increase engagement and motivation to <a href="http://onlinelibrary.wiley.com/doi/10.1002/oby.20958/full">change behaviour</a>. </p>
<p>Evidence that children’s behaviours are partially influenced by their genes also serves to alleviate the blame that often rests on parents. For example, our <a href="http://onlinelibrary.wiley.com/wol1/doi/10.1111/jcpp.12647/abstract">recent study</a> establishing considerable genetic influence on toddlers’ fussy eating could help to ease the guilt and frustration parents experience when dealing with an extremely fussy child. </p>
<p>Twin research has undoubtedly advanced our understanding of human nature and has revolutionised the way we discuss the complicated relationship between nature (genes) and nurture (environment). Twin research has also led to breakthroughs in molecular genetic research that has the potential to change the course of disease treatment. Twins remain a valuable tool for researchers to establish the lay of the land in relation to the complexity of human nature.</p><img src="https://counter.theconversation.com/content/67502/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Clare Llewellyn is an elected trustee for the UK Association for the Study of Obesity. </span></em></p><p class="fine-print"><em><span>Alison Fildes, Andrea Smith, and Moritz Herle do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Understanding how genes influence people’s behaviour doesn’t lead to fatalism.Andrea Smith, PhD candidate in Epidemiology/Public Health, Health Behaviour Research Center, UCLAlison Fildes, University Academic Fellow, University of LeedsClare Llewellyn, Lecturer in Behavioural Obesity Research, UCLMoritz Herle, PhD candidate in Epidemiology/Public Health, Health Behaviour Research Center, UCLLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/614412016-09-13T12:52:44Z2016-09-13T12:52:44ZWhy isn’t there a gene for depression?<figure><img src="https://images.theconversation.com/files/131123/original/image-20160719-8014-r3r874.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Dark skies.</span> <span class="attribution"><span class="source">Shutterstock</span></span></figcaption></figure><p>Depression is sometimes categorised as a mental, rather than a <a href="http://bjp.rcpsych.org/content/178/6/490">physical illness</a> – as though somehow mental health is different from physical health. But the brain is not a magical black box inside your head. It is an organ, just like the heart or lungs, made up of cells and supplied with blood. </p>
<p>These brain cells grow and develop, making contact with other brain cells to communicate with one another. They control all of our behaviour, emotions and actions. And sometimes they go wrong, leading to illnesses like depression.</p>
<p>Depression does <a href="http://edition.cnn.com/2009/HEALTH/03/24/suicide.hereditary.families/">run in families</a>, which could mean depression is genetic, an inherited condition. But families don’t just share genes – they also experience a similar environment. So how can we separate genetic influences on depression from environmental factors?</p>
<p>Twins are a useful tool for studying the influence of genes and the environment on many diseases. They can be identical (one egg splits to form two babies) and share the exact same genetic make up, or nonidentical (two separate eggs form two babies), sharing half of the genes. By studying genetically identical twins raised together in the same family environment, we can examine whether differences in depression rates might be due to non-shared environmental factors. </p>
<p><a href="https://genepi.qimr.edu.au/contents/p/staff/CV432.pdf">Analysis</a> in 2005 of a large number of twin studies indicated that around 40% of the risk of developing depression is genetic, and the remaining 60% associated with non-shared environmental factors. It is therefore too simplistic to say diseases like depression are either genetic or environmental. Both play their part.</p>
<p>Given that depression, like cancer, is not one single illness, it is perhaps not surprising that a single gene for depression has not been found. The variety of experiences also makes it difficult to find the right patient groups to study. People with depression may experience a single episode, or repeated episodes. They may develop depression in childhood, adolescence or later in life. Symptoms can vary, and include sleep disturbance, body weight changes, loss of interest in everyday activities, feelings of guilt and worthlessness, and suicidal thoughts. </p>
<p>The psychiatric <a href="http://www.med.unc.edu/pgc/">genetics consortium group</a> is a collaboration of hundreds of international scientists from 38 countries who share data from almost a million depressed patients worldwide. It <a href="https://www.sciencedaily.com/releases/2015/01/150107122305.htm">concluded in 2015</a> that most of the genetic effects found so far in the scientific literature are not very reliable or robust. But this does not mean genes are not involved. It just means we haven’t really done the right studies yet – because depression is such a variable condition. </p>
<p>Grouping together people with different diagnoses of depression also makes it hard to identify genetic contributions. Breakthroughs in understanding cancer in the last 20 years have really come from being able to <a href="https://theconversation.com/hard-evidence-are-we-beating-cancer-20870">separate breast cancer</a> from lung cancer from prostate cancer. But this is yet another challenge in depression research where symptoms and diagnoses are relied upon rather than a blood test for <a href="https://theconversation.com/a-blood-test-alone-wont-be-enough-to-test-for-suicide-risk-17331">specific markers</a> that can tell us that one patient has the same type of depression as another patient. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/131128/original/image-20160719-8005-16n6k44.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/131128/original/image-20160719-8005-16n6k44.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=300&fit=crop&dpr=1 600w, https://images.theconversation.com/files/131128/original/image-20160719-8005-16n6k44.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=300&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/131128/original/image-20160719-8005-16n6k44.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=300&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/131128/original/image-20160719-8005-16n6k44.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=377&fit=crop&dpr=1 754w, https://images.theconversation.com/files/131128/original/image-20160719-8005-16n6k44.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=377&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/131128/original/image-20160719-8005-16n6k44.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=377&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A DNA sequence or events?</span>
<span class="attribution"><span class="source">shutterstock</span></span>
</figcaption>
</figure>
<p>Another consortium of scientists known as <a href="http://www.well.ox.ac.uk/converge">CONVERGE</a> has had some success in identifying genes involved in depression in an ongoing research project. They published findings from a carefully defined subgroup of severely depressed Chinese women patients with recurrent major depressive disorder. Over 10,000 participants had their DNA sequenced, which revealed <a href="https://www.neuroscience.ox.ac.uk/publications/539174">small changes in two genes</a> that were linked to major depressive disorder: the LHPP gene and the SIRT1 gene. Both are involved in cellular metabolism (how cells generate energy and signals) but their potential function in depression is not yet known.</p>
<p>Then a <a href="https://www.sciencedaily.com/releases/2016/08/160801113648.htm">recent US study</a> of more than 300,000 people of European descent reported different genes being associated with depression. In contrast to the CONVERGE group, they tried to get as many people as possible to have their DNA analysed via a commercial genomics company. This resulted in a very different population being studied including people self-reporting with depression. It remains to be tested whether these latest “genes for depression” can be verified.</p>
<h2>Environmental factors</h2>
<p>Some people don’t like the idea that there are “genes for depression” because they think that means getting the illness is somehow inevitable. But this view overlooks the adaptable nature of our brains and our genes. Genes are like a set of very complex instructions which tell the cells what proteins to make, and their activity can change, helping brain cells adapt to different situations. And let’s not forget the other factor determining liability to develop depression: the environmental influences you are exposed to, <a href="http://sciencenordic.com/how-stress-can-cause-depression">such as stress</a>. </p>
<p>So is there a gene for depression? No, there isn’t. We don’t have genes for diseases, we have genes that encode proteins that serve functions in cells.
So does that mean if your DNA has the relevant change in your LHPP or SIRT1 genes that you are going to develop depression? No, it probably means you may have an increased risk to develop depression depending on the environment you experience.</p>
<p>In the same way that we think about heart disease, cancer, obesity as having multiple “susceptibility genes”, the same is probably true for depression. It is likely that there are small changes in a number of genes, each contributing a little bit to dysfunction of your brain cells, which can then lead to you developing depression. We are finding out more all the time – and that is surely an uplifting thought.</p><img src="https://counter.theconversation.com/content/61441/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Sarah Bailey has received research funding from the Wellcome Trust, Medical Research Council, the Royal Society and Janssen Pharmaceutica.
Sarah Bailey is a member of council for the British Association for Psychopharmacology and serves on the Education & Training Committee/ Animal Welfare and In Vivo Pharmacology group of the British Pharmacological Society.</span></em></p>It is an illness that runs in families – but there are other factors which go beyond inheritance.Sarah Bailey, Senior Lecturer, Neuropharmacology, University of BathLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/649352016-09-07T05:51:07Z2016-09-07T05:51:07ZWhy we should aim to deliver most twins at 37 weeks<figure><img src="https://images.theconversation.com/files/136805/original/image-20160906-25237-uao3mo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">You say tomato, I say potato.</span> <span class="attribution"><a class="source" href="http://www.shutterstock.com/pic-96938312/stock-photo-adorable-looking-twins-with-curly-hair.html?src=XJSsexz62yZFAwHLX6apvw-1-6">Shutterstock</a></span></figcaption></figure><p>Increasing numbers of mothers are now pregnant <a href="http://www.ncbi.nlm.nih.gov/pubmed/23046551">with twins</a>, mainly due to the use of assisted reproductive techniques such as IVF. These pregnancies can either be monochorionic, where the twins share the same placenta, or dichorionic, where each twin has its own and which is more common.</p>
<p>Twin pregnancies are often complicated by problems related to the placenta, maternal age, obstetric complications, and concerns about the growth of the twins. Complications can often happen in pregnancies near term that have otherwise gone well, with no single cause identified for why this happens.</p>
<p>Twin pregnancies are at high risk of stillbirths compared to mothers carrying just one baby. This <a href="http://www.scirp.org/journal/PaperInformation.aspx?PaperID=27145">risk increases</a> 13-fold if twins share the placenta, and five-fold when they have their own placenta. </p>
<p>Stillbirth risk also increases as the pregnancy advances so mothers are often delivered before their due date. But early delivery poses additional risks to the baby including neonatal death and other complications, often arising from being born prematurely. The optimal time for delivery believed to minimise risk to newborns varies and current national and international guidelines recommend delivery at different time points, with some including <a href="https://www.nice.org.uk/guidance/cg129">the UK</a> and <a href="http://www.ncbi.nlm.nih.gov/pubmed/21277672">France</a> advising delivery as early as 34 weeks if the twins share the placenta – six weeks before the expected date of delivery.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/136807/original/image-20160906-25266-1tidxxw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/136807/original/image-20160906-25266-1tidxxw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/136807/original/image-20160906-25266-1tidxxw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/136807/original/image-20160906-25266-1tidxxw.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/136807/original/image-20160906-25266-1tidxxw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=501&fit=crop&dpr=1 754w, https://images.theconversation.com/files/136807/original/image-20160906-25266-1tidxxw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=501&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/136807/original/image-20160906-25266-1tidxxw.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=501&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">Early post.</span>
<span class="attribution"><a class="source" href="http://www.shutterstock.com/pic-259726004/stock-photo-twin-newborns-in-a-basket.html?src=zKq1ukHPhtg5Y1PpjOKe6A-1-1">Shutterstock</a></span>
</figcaption>
</figure>
<p>In a new paper <a href="http://www.bmj.com/content/354/bmj.i4353">published in the BMJ</a>, we identified the optimal time to deliver mothers with twin pregnancies to reduce the risks of both stillbirth and neonatal death. The work involved a team of international research collaborators from the <a href="http://www.ncbi.nlm.nih.gov/pubmed/22893554">Global Obstetric Network</a>. We systematically reviewed all published and unpublished evidence on delivery times for twins, which compared the weekly risks of stillbirth when a pregnancy was continued to the following week, to the risks of neonatal death when delivered at that gestational week. </p>
<p>We identified 32 studies that provided data on about 35,000 twin pregnancies (29,685 dichorionic and 5,486 monochorionic). We found that in twin pregnancies where each has its own placenta, the risks of neonatal death from delivery was more or equal to the risk of stillbirth from continuing the pregnancy. However, when pregnancy was continued from 37 to 38 weeks, the risks of stillbirth appeared to be greater than neonatal death, resulting in 8.8 additional infant deaths per 1,000 pregnancies. </p>
<p>In twin pregnancies that shared the placenta, this risk of stillbirth appeared to be greater than neonatal death when pregnancy was continued from 36 weeks to 37 weeks, resulting in 2.5 additional deaths per 1,000 pregnancies. But this increase was not statistically significant. </p>
<p>We also provided estimates of other complications in twin babies such as respiratory distress syndrome, septicaemia, neonatal seizures, and admission to the neonatal intensive care unit for a complication. </p>
<p>Based on our findings, we recommend that delivery should be offered to mothers with twin pregnancies that have their own placentas (dichorionic) at 37 weeks of pregnancy. We did not find clear evidence to deliver twins with a shared placenta (monochorionic) before 36 weeks of pregnancy – two weeks after some guidelines – and so suggest that delivery should be considered at 36 weeks in these pregnancies.</p><img src="https://counter.theconversation.com/content/64935/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Shakila Thangaratinam does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Delivering twins is a balance between preventing pregnancy complications and the dangers of prematurity. New research suggests there is an optimum time.Shakila Thangaratinam, Professor in Maternal and Perinatal Health, Queen Mary University of LondonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/580522016-06-12T19:39:16Z2016-06-12T19:39:16ZGenes can have up to 80% influence on students’ academic performance<figure><img src="https://images.theconversation.com/files/125658/original/image-20160608-3497-1b1wmin.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Genes do have an influence over school performance. But they are never the full story. </span> <span class="attribution"><span class="source">from www.shutterstock.com</span></span></figcaption></figure><p><a href="http://dx.doi.org/10.1080/10888438.2013.800521">Research shows</a> that a student’s genetic makeup can have a strong influence on their academic performance. </p>
<p>Some interpret this as meaning there is little that can be done to help those who struggle academically – and that spending extra money on these students to help them succeed <a href="http://www.theaustralian.com.au/opinion/cash-cant-beat-genetics-in-class/news-story/6d62e16e5061602a25f6398f52eb94d6">is pointless</a>.</p>
<p>But is this the case? </p>
<p>A major misconception is that genes are destiny. This is wrong because genes are never the full story. </p>
<p>This is because environmental factors (“nurture”) also play a role in levels of academic achievement. Well-designed and well-delivered remediation can also help struggling students even in cases where genetic factors (“nature”) may be the source of the difficulties.</p>
<h2>What we know about genetic influence</h2>
<p>We know about strong genetic influences on academic skills primarily through the use of the <a href="http://link.springer.com/article/10.1007%2Fs10519-013-9598-6">twin method</a>.</p>
<p>This is where the genetic makeup of identical twins is compared with non-identical twins. </p>
<p>Evidence of genetic influence emerges if identical twins are more alike in terms of academic performance than non-identical (“fraternal”) twins. </p>
<p>Identical twins share all their genes, “fraternal” twins share half of their genes, but both types share homes and schools. </p>
<p>So <a href="http://www.ncbi.nlm.nih.gov/pubmed/16173891">researchers can estimate</a> the degree to which genes affect academic achievement over and above the effects of homes and schools: that is, they can estimate how much ability is inherited. And because non-identical twins can be opposite-sex, researchers can also identify if nature and nurture play out differently with males and females. </p>
<p>For the most part the same genes appear to affect boys and girls, and in general gender effects are in danger of being exaggerated in public discourse.</p>
<p>Studies with twin children have been conducted worldwide, including in <a href="http://link.springer.com/article/10.1007/s11145-006-9019-9">Australia, the US</a>, the <a href="https://kclpure.kcl.ac.uk/portal/files/35193441/Shakeshaft_Trzaskowski_McMillan_et_al_2013_PLoS_ONE.pdf">UK</a>, <a href="http://www.ncbi.nlm.nih.gov/pubmed/22700061">continental Europe</a>, <a href="http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0016640">Asia</a>, and <a href="http://link.springer.com/article/10.1007/s11145-006-9017-y">Africa</a>, with an emphasis on the core areas of literacy and numeracy. </p>
<p><a href="http://www.tandfonline.com/doi/abs/10.1080/10888438.2013.800521">Estimates of genetic influence</a> vary somewhat among subjects and locations, but range from near 50% to as high as 80%. The studies have used standardised tests as well as <a href="https://kclpure.kcl.ac.uk/portal/files/35193441/Shakeshaft_Trzaskowski_McMillan_et_al_2013_PLoS_ONE.pdf">school-administered tests</a>.</p>
<p>Less is known about creative and technical subjects, where particular talents clearly exist.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/125660/original/image-20160608-3492-1bw6ds0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/125660/original/image-20160608-3492-1bw6ds0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=414&fit=crop&dpr=1 600w, https://images.theconversation.com/files/125660/original/image-20160608-3492-1bw6ds0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=414&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/125660/original/image-20160608-3492-1bw6ds0.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=414&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/125660/original/image-20160608-3492-1bw6ds0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=520&fit=crop&dpr=1 754w, https://images.theconversation.com/files/125660/original/image-20160608-3492-1bw6ds0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=520&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/125660/original/image-20160608-3492-1bw6ds0.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=520&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Identical twins are more alike in terms of academic performance than non-identical twins.</span>
<span class="attribution"><span class="source">from www.shutterstock.com</span></span>
</figcaption>
</figure>
<h2>What about environmental influence?</h2>
<p>Twin studies can also parse environmental influence into factors that twin children mostly share, such as home socio-economic status (SES) and school attended. There are also those that are unique to each child in a twin, such as illnesses and, often enough, separate teachers. </p>
<p>Contrary to many people’s expectations, some shared factors such as family SES and school attended are relatively <a href="http://slatestarcodex.com/2016/03/16/non-shared-environment-doesnt-just-mean-schools-and-peers/">minor influences</a> on student differences once genetic endowment has been taken into consideration. </p>
<p>It is important to note, however, that some groups may show lower average levels of achievement due to adverse environmental circumstances such as <a href="http://www.aihw.gov.au/closingthegap/documents/issues_papers/ctg-ip01.pdf">poorer rates of school attendance and retention</a>. </p>
<p>Other groups may be affected by unusual environments, such as <a href="http://europepmc.org/abstract/med/26448503">heavy metal contamination</a> from mining and metals processing, which can be associated with lower NAPLAN scores. </p>
<h2>Educational interventions</h2>
<p>What works are well-designed, well-delivered and timely interventions that can help struggling children to reach or more closely approach normal-range levels. </p>
<p>These interventions are usually designed for <a href="http://psycnet.apa.org/journals/edu/106/1/46/">individuals or small groups</a> but have proven successful when implemented at <a href="http://aer.sagepub.com/content/43/1/137.2.abstract">school district level</a>. </p>
<p>We do not claim that compensating for genetic disadvantage is easy, but with the right <a href="http://www.scientificamerican.com/article/the-secret-to-raising-smart-kids1/">frame of mind</a> and sustained help with an emphasis on <a href="http://trove.nla.gov.au/work/8407677?">how the alphabet represents the sounds of speech</a>, plus supported reading practice, progress is real and rewarding. </p>
<h2>Funding implications</h2>
<p>This is why the conclusion that strong genetic influence makes additional spending pointless is too pessimistic. </p>
<p>It could be argued that if some children struggling with literacy or numeracy are doing so because of constraints on learning with biological origins, then extra funding delivered to these children is exactly what is needed. </p>
<p>This is especially so if we wish to counter increasing <a href="https://grattan.edu.au/report/widening-gaps/">gaps</a> between the best - and worst -performing students. </p>
<h2>Implications for the teaching profession</h2>
<p><a href="http://onlinelibrary.wiley.com/doi/10.1111/j.1751-228X.2007.00001.x/abstract">Some teachers</a> have been reluctant to acknowledge the role of genes in school performance, perhaps because of an aversion to biological explanations - so-called <a href="https://en.wikipedia.org/wiki/The_Blank_Slate">“biological determinism”</a> - and perhaps because of the false impression that if genes matter, teachers don’t. </p>
<p>Among other consequences, this has meant an overemphasis on the role of teacher skill and dedication in determining why some students prosper and others struggle. </p>
<p>There is <a href="https://theconversation.com/genes-arent-destiny-but-teaching-isnt-everything-either-10561">direct evidence from twins</a> that teacher differences are not responsible for much in the way of student differences in literacy. So teachers do matter in that they are the reasons why children know more at the end of the year or even the day. But our teachers are more uniformly effective than many give them credit for.</p>
<h2>The Colorado story</h2>
<p>It is unfortunate that in some education systems, such as in <a href="https://www.cde.state.co.us/educatoreffectiveness/overviewofsb191">Colorado</a> in the US, teacher employment and remuneration are tied to evaluations that give undue weight to student progress. </p>
<p>This ignores the fact that some students struggle because of biological constraints on learning that can be overcome to an encouraging degree, but only with special and adequate resources. </p>
<p>In the US, teacher morale is at an <a href="http://www.rawstory.com/2015/04/crisis-in-american-education-as-teacher-morale-hits-an-all-time-low/">all time low</a>, and in other places, including <a href="http://psycnet.apa.org/journals/edu/102/1/32/">Australia</a>, teachers are blamed by many in the media and politics.</p>
<h2>What is needed</h2>
<p>We need a more nuanced understanding of the factors that influence academic achievement, including the role that genes play.</p>
<p>At the same time, we need to avoid the unwarranted pessimism that can accompany acknowledgement of genetic influence, a danger that applies not only to attitudes toward academic development but to <a href="http://cdp.sagepub.com/content/24/5/399.abstract">mental and physical health</a> as well. </p>
<p>We need to take comfort from the existence of scientifically-grounded interventions, which in the hands of teachers with sufficient resources, can make a difference to the prospects of students who initially find the going in particular subjects tough.</p><img src="https://counter.theconversation.com/content/58052/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Brian Byrne receives funding from the Australian Research Council and the National Health and Medical Research Council.</span></em></p><p class="fine-print"><em><span>Richard Olson receives funding from the National Institutes of Health, U.S.A. </span></em></p><p class="fine-print"><em><span>Katrina Grasby does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Some students struggle because of biological constraints on learning. This can be overcome to an encouraging degree, but only with special and adequate resources.Brian Byrne, Chief Investigator, ARC Centre of Excellence in Cognition and its Disorders, Chief Investigator, NHMRC Centre of Research Excellence in Twin Research, and Emeritus Professor, School of Behavioural, Cognitive and Social Sciences, University of New EnglandKatrina Grasby, PhD, University of New EnglandRichard Olson, Professor of psychology and neuroscience director, Colorado Learning Disabilities Research Centre, University of Colorado BoulderLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/554792016-03-17T13:03:50Z2016-03-17T13:03:50ZHow does genetics explain non-identical identical twins?<figure><img src="https://images.theconversation.com/files/114707/original/image-20160310-26271-122kbus.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Identical twins share the same DNA.</span> <span class="attribution"><a class="source" href="http://www.shutterstock.com/cat.mhtml?lang=en&language=en&ref_site=photo&search_source=search_form&version=llv1&anyorall=all&safesearch=1&use_local_boost=1&autocomplete_id=&search_tracking_id=Y3Md7DHLru55fkq6uhwfpw&searchterm=DNA&show_color_wheel=1&orient=&commercial_ok=&media_type=images&search_cat=&searchtermx=&photographer_name=&people_gender=&people_age=&people_ethnicity=&people_number=&color=&page=1&inline=134698571">www.shutterstock.com</a></span></figcaption></figure><p>Newspapers recently went wild with a story about <a href="http://www.dailymail.co.uk/femail/article-3461832/First-identical-twins-different-skin-colour-born-UK.html">identical twin sisters with a difference</a>: they weren’t identical. Like all identical twins, Amelia and Jasmine arose from a single fertilised egg so have identical DNA, but somehow look different from each other. </p>
<p>One child has dark skin, black hair and brown eyes while the other has fair skin, light-brown hair and blue eyes. How is this possible? Have the doctors or scientists got it wrong? </p>
<p>No one has got it wrong. They really are identical twins. While this is a rare event – the doctors were as surprised as anyone – genetic and epigenetic research tells us that it is possible. In fact, it is likely to be the result of common biological processes that are going on inside all of us all of the time – although, typically, with less visually striking effects.</p>
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<h2>Types of twin</h2>
<p>Twins fall into two broad categories: <a href="http://www.twinsuk.co.uk/twinstips/4/140/twin-pregnancy--multiple-births/types-of-twins--identical-fraternal--unusual-twinning/">identical and non-identical</a>. Non-identical twins are the result of a mother releasing two eggs at once, both of which become fertilised by different sperm resulting in two separate zygotes which develop into two children. </p>
<p>Like most full siblings, non-identical twins share 50% of their DNA. Identical twins, however, result from a single fertilised egg (one zygote) separating into two parts during early cell division. It’s normal for a zygote to divide and it does this repeatedly during development to grow the many cells needed to make a full-term baby, whether that baby is a twin or not. But for singletons and non-identical twins these cells remain tightly packed together. If the cells separate early on in the process they can develop as two independent but twinned zygotes that share 100% of their DNA. Since DNA and the genes within it contain all the biological instructions for making us, this should result in two identical looking babies: identical twins.</p>
<h2>Imperfectly perfect</h2>
<p>We are taught in high school biology that when cells replicate their DNA and divide as we grow, they replicate our DNA <a href="http://www.educationscotland.gov.uk/highersciences/biology/unitone/dna/topicdetails.asp#replication">perfectly</a>. Actually, they do it almost, but not quite, perfectly. With each round of DNA replication and cell division our cells are accumulating a small number of changes in their DNA sequences. So even within our own bodies, our cells do not contain identical DNA. This is known as <a href="http://www.mdpi.com/2073-4425/5/4/1064/htm">somatic variation</a>. </p>
<p>Some of these changes give rise to common familiar features such as moles, some have been linked to diseases such as cancer, but the majority have <a href="http://www.sciencedirect.com/science/article/pii/S0960982214005466">no obvious effect</a>. The same process occurs as identical zygotes grow and divide and will result in numerous subtle genetic differences between identical twins most of which we barely notice. </p>
<p>Consequently, identical twins do not actually <a href="http://jmg.bmj.com/content/51/1/28.long">share 100% of their DNA</a> although they share very close to 100%. And when the differences are in the genes responsible for features such as hair colour, eye colour or skin colour, twins will have obvious and dramatically different looks.</p>
<h2>The role of epigenetics</h2>
<p>In addition to this, genes can be switched on and off in different cells. In fact, they have to be. If all of our genes were switched on all of the time in all of our cells then we would not be able to grow different tissue types and organs from a <a href="https://www.genome.gov/27532724">single set of biological instructions</a>. </p>
<p>One of the main processes switching genes on and off is an epigenetic process known as DNA methylation. By controlling which genes are on or off in any given cell, we are able to grow kidneys, heart, skin, etc and control how these cells behave and what they look like. </p>
<p>DNA methylation marks can be inherited across generations, but, equally, they can be altered by relatively short-term stimuli such as <a href="http://www.sciencedirect.com/science/article/pii/S1550413112000058">exercise</a> or <a href="http://physiolgenomics.physiology.org/content/46/23/851.long">nutrition</a>. More importantly, there is <a href="http://www.fasebj.org/content/12/11/949.long">evidence</a> that the genes involved in controlling eye, skin and hair colour are subject to this epigenetic control. </p>
<p>So whether they had a different experience in the womb – such as one twin receiving more nutrients due to a better connection to the placenta – or whether there was some chance epigenetic reprogramming, it seems likely that epigenetics will have a role in explaining the difference in the appearances of Amelia and Jasmine.</p>
<p>Although a few eyebrows may be raised when Amelia and Jasmine are described as identical twins, biologically they are as identical as any other pair of identical twins. The differences between them are just more visually striking.</p><img src="https://counter.theconversation.com/content/55479/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Colin Moran does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Just because identical twins have exactly the same DNA, doesn’t mean they will have the same physical features.Colin Moran, Senior Lecturer in Health and Exercise Sciences, University of StirlingLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/561642016-03-12T09:22:53Z2016-03-12T09:22:53ZExplainer: how can twins have different fathers?<figure><img src="https://images.theconversation.com/files/114852/original/image-20160311-11288-eybiic.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Shutterstock</span></span></figcaption></figure><p>The recent report from northern Hòa Bình province in Vietnam of twins born to two different fathers has been making headlines around the world. The father of the twins took the infants for DNA tests where it was revealed he was the biological father to just one of them - the other twin was <a href="http://www.bbc.co.uk/news/world-asia-35761281">fathered by another man</a>. How could a set of twins have different fathers?</p>
<p>This is an extremely rare occurrence in humans and is known as heteropaternal superfecundation. We don’t know exactly how often this occurs and cases only arise when suspicious family members request DNA testing. But one study estimated that it might occur in as many as <a href="http://www.ncbi.nlm.nih.gov/pubmed/7871943?dopt=Abstract&holding=npg">one in 400</a> (0.25%) twin births in the US. Another study reported that among non-identical twins whose parents had been involved in paternity suits <a href="http://www.ncbi.nlm.nih.gov/pubmed/1488855">the frequency was 2.4%</a>. </p>
<h2>Conception challenge</h2>
<p>For heteropaternal superfecundation to occur, the mother’s body must release two eggs during ovulation that are then fertilised by two sperm cells from two different men. The odds of one sperm fertilising an egg during one instance of intercourse are actually rather small. So the chances of two sperm cells from different males being successful are even smaller, relying on a culmination of timing and superb reproductive biology. A rare event indeed.</p>
<p>From the millions of sperm deposited during intercourse, only a few hundred or less reach the eggs. The sperm’s journey through the female reproductive tract is an arduous process and they they must circumnavigate the cervix, uterus and fallopian tubes to reach the eggs. At the same time they have to survive the harsh environment of the female reproductive tract and avoid the woman’s immune response, which sees white blood cells target sperm cells as invaders.</p>
<p>Fertilisation is also a matter of timing. The ovulated egg is available for a short window (12-24 hours) and so the sperm must be present in the fallopian tube during that time for fertilisation to occur. In the case reported from Vietnam, the woman would have had to have intercourse with two different men over a short period - within at least a day before or after ovulation - for both eggs to be fertilised. </p>
<p>Almost <a href="bit.ly/1RbM2by">one in 100 births</a> in the UK and US are to non-identical or “dizygotic” twins, although the global frequency varies widely with factors such as genetics, nutrition statue and BMI all playing a role. Rates also increase considerably with maternal age, probably because of changes in reproductive hormone levels. Women aged 35-39 are <a href="https://www.ncbi.nlm.nih.gov/pubmed/10336022">four times as likely</a> to have dizygotic twins than those aged 15-35.</p>
<h2>Copulation competition</h2>
<p>Although heteropaternal superfecundation is rare in humans, it is <a href="http://www.ncbi.nlm.nih.gov/pubmed/20394608">not uncommon in nature</a> and has been reported in many animal species including dogs, cats, cows, mink and rodents. What’s more, in many species that have multi-pair copulations, the males have developed a variety of strategies to ensure their sperm reach the egg.</p>
<p>This can include evolving strange penile structures to scoop out rival sperm (<a href="http://www.nytimes.com/2011/10/18/science/18dragonfly.html?_r=0">as in dragonflies</a> and damselflies), or damage the female, thus preventing subsequent mating (referred to as <a href="https://theconversation.com/invertebrates-inject-a-bit-of-romance-during-sex-by-stabbing-each-other-24154">traumatic insemination</a>. This phenomenon is referred to as “<a href="http://www.sciencedirect.com/science/article/pii/S0960982204000284">sperm competition</a>”. It has even been suggested that the shape of the human penis evolved to function as a displacement device to remove any semen <a href="http://www.sciencedirect.com/science/article/pii/S1090513803000163">deposited by a previous male</a>.</p><img src="https://counter.theconversation.com/content/56164/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Michael Carroll does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>A set of twins in Vietnam have been found to have different fathers.Michael Carroll, Senior Lecturer in Reproductive Science, Manchester Metropolitan UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/545042016-02-12T10:04:50Z2016-02-12T10:04:50ZWhy a bit of grit won’t get children higher grades<figure><img src="https://images.theconversation.com/files/111163/original/image-20160211-29188-5v1e5v.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Where will true grit get you? </span> <span class="attribution"><span class="source">Gianni Caito/www.shutterstock.com</span></span></figcaption></figure><p>Children differ widely in how well they do in exams, and researchers are still trying to understand which factors can predict or explain these differences, and if genes play a part. Now <a href="http://psycnet.apa.org/psycarticles/2016-06824-001">our new study</a> has found that contrary to previous research, there is little to suggest that “grit” – or a child’s perseverance and passion for long-term goals – has an impact on their exam results. </p>
<p>Previous <a href="http://www.ncbi.nlm.nih.gov/pubmed/19254083">research</a> has shown that a child’s personality can predict a significant, although modest, proportion of the differences between children’s grades at school. For example, a link between conscientiousness and school achievement can explain around 4% of the differences in children’s grades. </p>
<p><a href="http://www.ncbi.nlm.nih.gov/pubmed/24550863">Research</a> has also shown that grit is associated with academic achievement and <a href="https://www.sas.upenn.edu/%7Educkwort/images/Grit%20JPSP.pdf">life outcomes</a> – even when intelligence and other personality factors are controlled for.</p>
<p>Yet, most of the research to date has used restricted samples, for example undergraduate students, <a href="http://www.sas.upenn.edu/%7Educkwort/images/publications/truegrit.pdf">teachers</a> and children who were <a href="http://spp.sagepub.com/content/early/2010/10/01/1948550610385872">finalists</a> in spelling competitions. Much less is known about the association between grit and academic achievement in the general population. It is also largely unknown why people have such different levels of grit.</p>
<p>To answer this question and explore the relationship between grit and academic achievement, our research used a sample of 4,500, 16-year-olds (from 2,300 pairs of twins) who are part of the UK <a href="http://www.teds.ac.uk">Twins Early Development Study</a>. By studying the origins of grit and its association with GCSE exam results, we found that grit actually has little impact on exam results.</p>
<h2>Measure your true grit</h2>
<p>We used <a href="https://sites.sas.upenn.edu/duckworth/pages/research">the “Grit-S” questionnaire</a> to measure each teenager’s level of perseverance and how consistently they held an interest in a task. To measure their perseverance, the participants rated the extent to which they agreed with statements such as “setbacks don’t discourage me”. To measure how consistently they maintained an interest in something, we asked them whether they agreed that: “I have a difficulty maintaining my focus on projects that take more than a few months to complete.” </p>
<p>We also gave them a questionnaire to assess their level of each of the <a href="http://www.ncbi.nlm.nih.gov/pubmed/16672728">“big five” personality traits</a>: extroversion, agreeableness, neuroticism, openness and conscientiousness.</p>
<p>By using twins in our study we were able to look at why people differ in the amount of grit they have – and if genes played a part. We compared identical twins, who share 100% of their genes, to non-identical twins, who share on average 50% of the genes that vary between people. </p>
<p>This meant we could estimate whether differences we observed were down to differences in the twins’ DNA sequence (known as heritability), their shared environment (such as school and family influences) or their non-shared environment – such as different friendship groups. If identical twins are more alike on a particular trait, such as grit, than non-identical twins, we can infer that this trait is influenced by their genes. </p>
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<p>Our results showed that whether or not a person has more or less grit is substantially influenced by their DNA – and explains around a third of the differences between people’s level of grit. We showed that grit is highly similar to other personality traits, showing substantial genetic influence and no influence of shared environmental factors. </p>
<h2>Weak link with exam results</h2>
<p>Leaving genes aside, when we looked at the relationship between personality and academic achievement, we found that the big Five personality traits – mainly conscientiousness – explained 6% of the differences between exam results of the 16-year-olds in our study. But after controlling for these personality traits, grit on its own did little to influence academic achievement, explaining only an additional 0.5% in people’s GCSE results. </p>
<p>These results should warrant concern given education policy directives in the US and in the UK, which emphasise the importance of grit and <a href="https://theconversation.com/schools-can-teach-character-but-what-sort-of-person-do-we-want-to-produce-23201">character education</a>. It’s clear from our study that more research must be done before concluding that a certain type of teaching or classroom intervention are beneficial for academic achievement or other life outcomes. </p>
<p>We’re not suggesting that it’s impossible to teach children to be grittier, or denying that it’s beneficial to build habits of working hard or continuing in the face of adversity. But our findings suggest that grit is not a good way of predicting whether a child will get good grades. So while increasing grit or perseverance could have long-term benefits for children, it is unlikely to improve their academic achievement.</p><img src="https://counter.theconversation.com/content/54504/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Kaili Rimfeld receives funding from UK Medical Research Council (MRC). </span></em></p>A new study has found that having perseverance and long-term goals won’t boost a child’s exam results.Kaili Rimfeld, PhD Student, Institute of Psychiatry, Psychology & Neuroscience, King's College LondonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/450592015-07-23T13:17:27Z2015-07-23T13:17:27ZThe same genes influence exam results across a range of school subjects<p>Could it be that genetic differences can affect how well children perform in exams? Our research suggests that this may well be the case and that individual differences between children are, to a large extent, due to the inherited genetic differences between them that predisposes them to do well academically, whatever the subject. </p>
<p>We also found that there is shared genetic influence across a range of subjects, even after controlling the exam results for general intelligence.</p>
<p>It goes without saying that children’s exam results at the end of compulsory education play a significant role in their future education and career paths. And it is also reasonable to assume that schools play a major role in school achievement. But children differ in educational achievement within the same school – and even the same classroom. This suggests that factors other than school or classroom differences explain the wide variation in pupils’ exam results. </p>
<p>Our new research, <a href="http://www.nature.com/srep/2015/150723/srep11713/full/srep11713.html">published in Scientific Reports</a>, examined the GCSE results, using classical twin method, that compares the correlations between identical and non-identical twins, and found that individual differences in exam results are to a large extent explained by the inherited differences in children’s DNA sequence. </p>
<p>We also found that many of the same genes influence achievement across a range of subjects – so, children who tend to do well in one subject tend to do well in others, largely for genetic reasons. </p>
<p>Previous research using data from the <a href="http://www.teds.ac.uk/">Twins Early Development Study</a> (TEDS), found that there is substantial heritability for educational achievement in early and middle school years. Heritability is a population statistic, which describes the extent to which differences between children can be explained by the differences in their DNA, on average, in a particular population at a particular time. </p>
<p>So, for example, a heritability of 90%, means that 90% of individual differences observed in a group of people for a trait are explained by genetic differences between them and 10% explained by environmental factors. What it doesn’t tell us, is anything about an individual.</p>
<p>We already knew, based on our <a href="https://www.teds.ac.uk/downloads/SRCDcomplete.pdf">research which was published in 2007</a>, (based on a UK representative sample of 7,500 twins pairs who were tested at the ages of 7, 9, and 12) that the average heritability for literacy and numeracy is almost 70%. In other words, more than two-thirds of the variation seen in academic test results is explained by genetic differences between children. </p>
<p>Further <a href="http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0080341">research</a> from 2013 also found that educational achievement, as measured by standardised exams (GCSEs), at age 16 is also substantially heritable, with genetic factors explaining about 60% of the variance in results of the core subjects of English, mathematics and sciences.</p>
<h2>How genes influence achievement</h2>
<p>Our new study sought to determine whether the high heritability of core academic subjects also extends to various other subjects, such as history and geography, which involve more fact-based knowledge – or art, music and drama, which are more subjective subjects.</p>
<p>We analysed achievement data from the twins in TEDS to assess the extent to which genetic factors influence various school subjects – and, in particular, GCSE exam results. </p>
<p>We found that genes explain a larger proportion of the differences between children across different subjects (54-65%) than shared environmental factors, such as home and school environment combined (14-21%). </p>
<p>However, it’s important to stress that heritability is a population statistic and this does not mean that genetics explain 54-65% of a single child’s school achievement. But it does indicate that differences in academic exam results are, to a large extent, explained by differences in people’s DNA. </p>
<p>Our study indicates that this substantial heritability for school exams is not explained by intelligence alone, as heritability for GCSE grades for all subjects remained substantial even after statistically removing the intelligence scores from the exam results. This finding is in line with <a href="http://www.pnas.org/content/111/42/15273.abstract">our previous research</a> in which we found a similar result for the mandatory subjects of English, maths and science.</p>
<p>We <a href="https://theconversation.com/how-genes-can-influence-childrens-exam-results-32535">had also found</a> that heritability of GCSE exam results involves the joint contribution of <a href="http://www.pnas.org/content/111/42/15273.abstract">many other factors</a>, including children’s self-efficacy, or pupil’s belief in his/her abilities, behavioural problems, personality traits, well-being, and their perceptions of school environment – as well as their intelligence.</p>
<p>Although our results cannot be applied directly to classroom teaching right now, they do, however, add to the growing knowledge of why children differ so widely in educational achievement.</p>
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<img alt="" src="https://images.theconversation.com/files/89365/original/image-20150722-1418-cpbxyv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/89365/original/image-20150722-1418-cpbxyv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/89365/original/image-20150722-1418-cpbxyv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/89365/original/image-20150722-1418-cpbxyv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/89365/original/image-20150722-1418-cpbxyv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=501&fit=crop&dpr=1 754w, https://images.theconversation.com/files/89365/original/image-20150722-1418-cpbxyv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=501&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/89365/original/image-20150722-1418-cpbxyv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=501&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Down to DNA?</span>
<span class="attribution"><span class="source">DNA squence via Gio.tto/www.shutterstock.com</span></span>
</figcaption>
</figure>
<h2>Same genes, range of subjects</h2>
<p>Our new results also indicate that achievement across a wide range of academic subjects including English, mathematics, science, humanities, second languages, business and art are influenced by many of the same genes.</p>
<p>This shared genetic influence is, to a large extent, independent of intelligence, suggesting that there is a genetically driven “general academic achievement factor”. This means that its largely down to genetic reasons that children who tend to do well in one subject also tend to do well in others even when different levels of intelligence is controlled. </p>
<p>Our findings could also facilitate molecular genetic research that aims to identify the genes responsible for academic achievement by focusing on achievement across different subjects, rather than focusing only on a specific subject such as mathematics or English.</p><img src="https://counter.theconversation.com/content/45059/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Kaili Rimfeld receives funding from UK Medical Research Council (MRC)</span></em></p>Children who tend to do well in one subject tend to do well in others, largely for genetic reasons.Kaili Rimfeld, PhD Student, Institute of Psychiatry, Psychology & Neuroscience, King's College LondonLicensed as Creative Commons – attribution, no derivatives.