Is the Y Chromosome Doomed?

 

   Humans store their genes in 23 pairs of chromosomes, 22 of which are identically matched. The 23rd is a two-sided biological coin—twin Xs mean you’re female; an X and a Y, male. Chromosome pairs often trade bits of DNA in a process called recombination, the purpose of which is to keep genes functioning properly. Talk of men’s path toward extinction began in the late 1990s, when it was discovered that the human Y chromosome, which is stumpy compared with the X, does not share enough genetic material with the X to practice recombination. Left without a way to renew damaged genes, the Y would continue to degrade and would eventually disappear, geneticists announced. They slapped an expiration date on the male half of the species of sometime in the next 5 to 10 million years. To get a perspective on this prediction, scientists looked to our closest genetic relatives—the chimps. Because humans and chimpanzees shared a common ancestor 6 million years ago, geneticist David Page of the Whitehead Institute for Biomedical Research in Cambridge, Massachusetts, studied how the chimp Y chromosome and its human Y counterpart have evolved differently in the intervening years. What he found surprised him: The chimp Y chromosome is far more degraded than the human Y chromosome. Page and his colleagues speculate that chimps’ promiscuity—females mate with multiple partners—has led to enhancement of the Y genes that produce sperm, to the detriment of other genes. Among chimps, “there are sperm wars going on. Each male is trying to pass his own genes down,” says Jennifer Hughes, who coauthored the study. Neglected, the chimp Y chromosome’s nonreproductive genes have declined.

    

   The Whitehead Institute scientists think that although the human Y chromosome also lost genes at first, in recent eons it has been relatively stable. The human Y has eluded the chimp Y’s fate, they suggest, because humans are largely monogamous. Human sperm don’t face the same competition as chimps’, so there isn’t as much pressure on the human Y to produce good sperm. Not all geneticists are convinced that the human Y has stopped deteriorating. Jenny A. Marshall Graves of the Australian National University in Canberra, believes that the Y chromosome’s days are numbered. “The human Y has been degenerating since it was born, 300 million years ago,” she says. And so the controversy continues. Rest assured, though; the Y chromosome—and the guys —will be around for a while.