Science / Medicine : Isolated Gene Proven to Determine Mammals’ Sex

Male mice have been created from female embryos by the injection of a single gene into fertilized eggs, confirming that the gene is the long-sought “sex trigger” that determines gender, British researchers reported last week in the journal Nature. Two “female” embryos developed male sex organs, and a third grew up to show normal male mating behavior, although all were infertile. The gene appears to work by regulating the activity of other genes involved in development.

In mammals, embryos develop as females unless they contain the Y chromosome. British scientists reported a year ago that they thought they had discovered the gene, called Sry, that triggers the process of making a male.

The key event is the creation of testes rather than ovaries from a particular bit of tissue in the embryo. Scientists injected Sry into 158 fertilized eggs, implanted them in mice and let some of the resulting embryos develop for two weeks. At that point they could check for development of testes.

Most of the embryos they tested were not affected and emerged as normal males or females. Two were embryos that lacked the Y chromosome but developed normal testes anyway because of the presence of the Sry gene. The researchers also let 93 embryos proceed through birth. One of these animals had no Y chromosome but did carry the Sry gene. It appeared outwardly to be a male and showed normal mating behavior.