Making Men

What makes a man? Biologists have long known that the answer lies not in what but in Y. To create a male child, a father's sperm must carry a Y chromosome to fertilize a mother's egg, which always bears an X chromosome. But the site of the specific gene on Y that determines maleness has been elusive. Last week, though, scientists in Britain announced in Nature that they have identified a section of DNA that apparently directs the development of the testes, the male reproductive glands. The gene is being called SRY for sex-determining region Y.

Scientists from London's Imperial Cancer Research Fund studied DNA from four men with an abnormal set of chromosomes: each bore an XX pairing that had a small piece of Y attached to one of the chromosomes. Segments of DNA cloned from the Y fragment were compared with genetic material from a wide range of male and female mammals, from chimps to tigers. Only one segment, which contained the SRY gene, was present in all the males and absent in all the females. Working with the ICRF team, London's Medical Research Council scientists showed that XY mice, which are female despite a Y chromosome, are missing part of the SRY gene.

Still, SRY is just one element in the mystery of maleness. "Sex determination involves a whole pathway of genes," observes MRC researcher Robin Lovell-Badge. "Now we have an entry point."