THE X IN SEX

The author of Making Babies (2001) takes a lively, witty tour of the X chromosome, creator of “a delicious asymmetry between men and women.”

Bainbridge (Comparative Anatomy and Physiology/Royal Veterinary College, London) begins by introducing an embryo that has not yet become either a boy or a girl baby and demonstrating how, for humans, the specialized X and Y sex chromosomes come into play. The default sex of an embryo is female; the presence of a Y chromosome is necessary to turn it into a male. While at first Y might seem the more powerful of the sex chromosomes, it is nearly empty of genes and almost incapable of doing anything other than control sex. Its counterpart, X, however, has profound effects on our lives and well-being. Bainbridge poses two questions: How do males cope with having just one X chromosome? How do females cope with having two? In a chapter on sex-linked diseases bearing the inspired title “The Duke of Kent’s Testicles,” he relates how hemophilia, a blood-clotting disorder transmitted by a damaged X chromosome, spread through certain royal families of Europe. Whereas girls inheriting this damaged X are protected by their spare, undamaged X chromosome, boys, having but a single X, become the victims of a life-threatening disease. Female bodies, on the other hand, says Bainbridge, lead a kind of double life owing to the X chromosome. Except for the germ cells, which later give rise to eggs, every cell in the female embryo switches off one of its X chromosomes, apparently at random. This makes every female body a complex mosaic: half the cells have an X chromosome from the mother, half from the father. Within this entertaining and informative, if slightly too brief account, the author intersperses two essays on the biology of the X chromosome; an epilogue considers some of the ramifications of sex selection.

A fine demonstration of science made accessible. (4 line drawings)