Tug-of-War Between Two Genes May Determine Sex of Embryos

Two genes lock in a tug-of-war to determine whether a mammal embryo will become a boy or a girl, British researchers report today in Nature. One of the genes, called Sry, has long been known as the master switch that makes an embryo become male. The new work suggests that a second gene, Dax1, tries to block its effect.

It almost always fails. So embryos with one Y chromosome, which carries the Sry gene, and one X chromosome, which carries Dax1, normally develop as males, while those with two X chromosomes become females. But in rare cases, the new study suggests, embryos get an extra copy of the Dax1 gene. And when two Dax1 genes gang up on the single Sry gene, the competition goes the other way, and the embryo becomes a female physically although it is male genetically. The study is an important step toward understanding how genes work together to produce either a male or a female.

Compiled by Times medical writer Thomas H. Maugh II