In humans and other mammals, sex is determined by the presence or absence of Y chromosome. If Y chromosome is present, it will channel the genital ridge of the embryo into the pathway of testis development, while in the absence of Y chromosome ovaries develop. Once testes have formed, they secrete anti-Müllerian hormone and testosterone, which masculinize the reproductive tract. By contrast, the female reproductive tract develops in the absence of fetal gonadal hormones. Testis development is brought about through the action of the sex determining region located on the short arm of the Y chromosome (Sry gene), but correct doses of other genes on autosomes as well as the X chromosome, are also required. Sry appears to be widely expressed in human fetuses, suggesting the possibility that its influence on development is not confined to the testes. There is additional evidence of a difference in developmental rates between XY and XX cleaving embryos, in which Sry and another gene in the sex-determining region named Zfy, for the zinc finger protein it encodes, are already expressed. These findings are consistent with the possibility that Y-chromosomal genes affect somatic sex differences prior to the formation of steroid hormones.
Biomedical Reviews 1997; 7: 75-83.
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