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What’s the genetic disease risk for children of related couples?

Marriages between people who are related is more common than you might think. Unlike what many people think, their offspring are not doomed to birth defects or medical problems. In fact, unless they both…

Children of people who are related are not doomed to birth defects or medical problems. Tiago Fernandes

Marriages between people who are related is more common than you might think. Unlike what many people think, their offspring are not doomed to birth defects or medical problems.

In fact, unless they both carry the same gene mutation, the couple’s chance of having a healthy child is almost as high as any other couple. Let’s examine why, through the story of one couple.

And baby makes three

Maria is planning a pregnancy with her partner Max. They visit their doctor to discuss family planning and pregnancy health, and during questioning, they disclose that they’re first cousins. Their doctor refers them to a clinical genetics service for further advice.

What risks do they face, if any? And what information would be requested by the clinical genetics service?

In multicultural Australia, marriage between family members does occur, most commonly between first or second cousins. From a medical perspective, Mary and Max have several issues to consider.

The first of these relates to their exact genetic relationship. Genetically speaking, the closer one is to a family member, the more genes will be shared. Monozygotic (identical) twins have the same genetic make-up and share 100% of their genes. A parent and child share half their genes, as do siblings. An uncle and his niece, or an aunt and her nephew (a second-degree relationship) share a quarter of their genes.

Maria and Max, being cousins (a third-degree relationship), share an eighth of their genetic make-up. This being the case, what are Maria and Max at risk of?

Receiving the same faulty genes from both parents puts a child at risk of having a genetic disease. Ryan Croson

As they share a significant proportion of their genes, the couple are at risk of having a child with an autosomal recessive condition. This kind of condition is caused by having a “double dose” of a faulty gene.

We have two copies of every gene (for most genes) – one inherited from our father, and one from our mother. For many genes, our body can cope with just a single working copy, but when both copies are faulty, the person gets an autosomal recessive disease. Examples of such diseases include cystic fibrosis, thalassaemia (diseases of the blood), and spinal muscular atrophy.

Calculating risk

Most of us carry a handful or so of faulty recessive genes so marrying within your family increases your chance of “meeting” someone else with the same faulty recessive genes as you. And working out the degree of risk to Maria and Max’s offspring depends on whether or not they have a known family history of an autosomal recessive condition.

In the genetics clinic, a medical geneticist would ask them about the health of family members going back several generations, and draw a detailed family tree. Reports about other family members might need to be verified to establish an exact diagnosis.

If Maria and Max do have a family history of an autosomal recessive condition, such as thalassaemia, their degree of risk could be calculated based on who the affected individual was. Depending on the exact condition, testing could be offered as carrier screening. In the case of thalassaemia, for instance, Maria and Max could be offered a blood test to look for changes in their blood cells that might indicate that they’re carriers of the thalassaemia gene.

But if they don’t have any family history of an autosomal recessive condition, the medical geneticist would have to rely on risk estimates based on population data and general experience. We know (based on Victorian data) that approximately four in 100 couples will have a baby with a birth defect, which may be mild or severe. First-cousin marriages add extra risk to this, resulting in an approximate doubling of the background risk.

Without a family history of an autosomal recessive condition, Maria and Max have an 8% chance of having a child diagnosed with a problem after birth. In other words, their chance of having a healthy baby is greater than 90%, a figure that most people find quite reassuring.

This figure is not too different to the general population risk of having a baby with a birth defect. Most related couples accept this risk and focus instead on general measures to have a healthy baby, such as taking folate, losing weight, and reducing their intake of alcohol and cigarettes.

For advice on this topic or if you have concerns about a possible genetic condition in your family, contact the Victorian Clinical Genetics Services or your local clinical genetics service.

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5 Comments sorted by

  1. Tony P Grant

    Neo-Mort

    Not having known anybody that has married their cousin (direct relation) it just seems "strange" and we were brought up to believe that was "pretty sick stuff".

    Over the last decade or so I have read and been told of a "sub-group" within our society that actually practise "keeping it all in the family" based on strong larger family control.

    When we talk about "middle eastern folk" their are many "tribes" those backed by the USA as well!

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  2. Hilde Rombout

    retired psychologist/teacher

    Thank you for your informative article, Tiong Tan. I am one of those people who married a first cousin, and though my children were born healthy, there are mental problems in the extended family (2 of my sisters had children with mental problems who icommitted suicide) as well as in my own immediate one ( i have a child with schizophrenia, who has attempted suicide quite a few times and another child who by all accounts has committed suicide). I wonder if mental problems are heriditary and whether children of first cousin couples are at greater risk.

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  3. Keith Hammond

    Retired technician FAO of the UN

    As a retired Animal Geneticist I consider this article, based as it is solely on 1900 Mendelian Genetics and simplistic coancestry principles, is more a rationale for the existence of the Genetics consulting business rather than providing sound information to individual families and society.

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  4. Arthur James Egleton Robey

    Industrial Electrician

    My understand is that some genes are recessive.
    Some genes are dominant.
    Some recessive genes are pathological.
    Some recessive genes are benign or even beneficial.
    Some dominant genes are pathological. (I'll get back to that one with an example.)
    Some dominant genes are benign or even beneficial.
    The chances of recessive genes expressing themselves increases among sibling crosses, because both the siblings carry a suppressed recessive gene.
    If that recessive gene is pathological, then we have…

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  5. Stiofán Mac Suibhne

    Contrarian / Epistemologist

    This article is highly suspect. The dumbing down is excessive and the focus of the material so narrow as to potentially mislead. Not referring to large populations and customs of consanguineous marriage is a glaring omission.

    It's quite some time since I was concerned with such matters but its clearly undermining the author's credibility that the Cypriot experience of reducing thalassemia by challenging conventions of consanguineous marriage is not referred to or the very stark contrast between…

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