A study in mice shows it is possible to delete extra chromosomes in a range of conditions that are associated with infertility, including Down's syndrome.
Unlike other chromosomes, the X chromosome is inactivated in nearly all cells in women – and genes on it are active in very few tissues.
What produces the differences between men and women? Are they trivial or profound? Are they genetic or environmental, or both? And are men really closer genetically to chimpanzees than to women?