Making a man is the complex task of culture. Making a male-- nature’s job--is also none too simple. The first requirement is the sex- determining gene on the Y chromosome, called the SRY gene: it enables a fetus to build testes, which manufacture the sine qua non of maleness, testosterone. But testosterone alone is not enough; a second hormone is required to produce a normal, fertile male. Molecular geneticist Richard Behringer and his colleagues at the University of Texas M. D. Anderson Cancer Center in Houston have shown just how important this hormone is in male mice. When it is absent, the males develop as infertile pseudohermaphrodites--meaning they have the internal reproductive ducts of a female as well as those of a male. They look like a regular male, says Behringer. You wouldn’t know unless you look inside that something is funny.
In mammals, Behringer explains, male and female embryos ...
