The SRY gene

In the early stages of development, human embryos have no external genitalia. Whether they develop testes or ovaries depends on the presence and activity of a particular gene on the Y chromosome. This gene, called the sex related Y gene (SRY gene), was discovered in 1990 when geneticists were studying some interesting people: men who had two X chromosomes and women who had one X and one Y chromosome. Microscopical examination of the sex chromosomes of these people revealed that the XX males had a very small piece of Y chromosome in their X chromosomes, whereas this piece was missing from the Y chromosome of the XY females. The geneticists found the SRY gene within this small piece of Y chromosome.

The SRY gene codes for a protein called testis determining factor.This switches on other genes, causing the embryo to develop male structures. The testes develop and androgens (hormones which promote the development of male sexual organs and secondary sexual characteristics) are secreted. At about 16 weeks, an embryo with the SRY gene begins to produce immature sperm. In addition to stimulating male structures to grow, SRY suppresses the development of female structures by activating a gene on chromosome 19. This activation leads to the production of a protein called Mullerian- inhibiting substance, which destroys female structures early in their development. Lack of testis determining factor results in the development of female genital organs. Therefore, all embryos are female unless active testis determining factor makes them male.