Vive la Différence!

The human genome is made up of forty-six chromosomes, the rod-like structures that reside in the nucleus of every cell. These chromosomes carry all of our genes, which, in turn, are made of DNA. Two of these chromosomes, called the X and the Y, are different from the rest: they are “sex chromosomes.” Men carry one X and one Y chromosome, while women carry two X chromosomes. All the obvious physical differences between the sexes ultimately spring from this humble difference in chromosomal constitution.

During the last few years, real progress has been made in our understanding of the sex chromosomes and we now know much more about our X and Y than we did a mere decade ago. In 2003, for example, essentially the entire stretch of DNA carried on the human Y chromosome was decoded, revealing the number and, in many cases, identity of the genes that make up this seat of maleness. More important, owing to a breakthrough that occurred in the early Nineties, biologists now understand just how sex is decided in human beings—geneticists identified the master “switch gene” that determines whether an embryo will develop into a male or a female.

These discoveries might seem surprisingly recent. In view of the confident pronouncements in the medical press about all things having to do with sex and gender (homosexuality, for example, was said to be genetically determined), you’d be forgiven for assuming that the biology of how a human being becomes a boy or a girl has long been understood. To be fair, though, there were good reasons for the slow progress. How sex is determined represents a rare problem in which the study of simpler organisms like fruit flies led biologists astray. Sex determination in human beings specifically and in mammals generally doesn’t work the way it does in most of the species that geneticists like to study. Moreover, genetic studies in human beings are simply harder to perform than those in species like the fruit fly: a generation is more like two decades than two weeks, and we can’t dictate who mates with whom, an ethical constraint that doesn’t arise with flies.

Although the three books discussed here cover much of the same ground, one stands out from the rest. Bryan Sykes’s Adam’s Curse is both far more ambitious, and controversial, than Steve Jones’s or David Bainbridge’s book. Sykes, a professor of genetics at Oxford University and author of the best-selling The Seven Daughters of Eve, sounds loud alarms about an impending biological crisis involving the Y chromosome. He also makes bold sociobiological claims about the effect of the Y chromosome on our lives. Because Sykes is a leading researcher in the study of sex chromosomes (not to mention a science adviser to the House of Commons), his pronouncements merit special attention.


Sykes begins his book with the discovery of the master gene that decides sex in human beings. For decades, biologists understood that human beings have so-called Y-dominant sex determination. Roughly speaking,…

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