There is growing evidence to suggest that the presence of a single gene on the Y chromosome, which also codes for the expression of the H‐Y antigen, causes the primordial gonad to develop structurally as a testis: in the absence of this gene, an ovary is formed. Whether the gonad subsequently becomes capable of producing viable gametes depends on the sex chromosome makeup of the germ cells that migrate into it; XY or XO germ cells seem to be capable of developing into either oocytes or spermatocytes, depending on the environment, whereas germ cells with two X chromosomes can only develop as oocytes, and will not survive in a testicular environment. The H‐Y antigen probably plays no role in the sexual differentiation of the germ cell.There is another gene on the Y chromosome that determines testis size, and testes: body weight ratios vary widely in different mammals. Amongst the higher primates the gorilla has the lowest ratio (0.018%), man is intermediate (0.079%) and the chimpanzes have the highest (0.269%). Testis size is predominantly determined by the amount of tubular tissue, and appears to be closely related to copulatory frequency, and hence to the mating behaviour of the species. It is important to take these marked species differences in testis size into account when using primate models for human male contraceptive research and development.
Read full abstract