The Role of Genes.
The genes and their products play an important role in the process of sexual differentiation. Some of the genes, which are involved in the process of sex differentiation and development of gonads are discussed below.
H-Y/Antigen: We are aware that the Y-chromosome that carries the factors responsible for the development of male sexual characteristics. Gene for H-Y antigen is significant in sex differentiation. During a transplantation antigen in mice in 1955, it was found the gene that code for the H-Y antigen is male specific (holandric) and it is located on the short arm of the Y chromosome. It serves as an initial signal for primary sexual differentiation and essential for the development of testis in mammals. It is believed that the antigen has been preserved throughout evolution. Its presence was detected by adding anti-H-Y antibodies to white blood cells. It was observed that these H-Y antigen bind to the antibody on the surface of the cell. In amphibians, where the female is the heterogametic sex, this H-Y antigen is expressed.
Sxr Gene: The evidences for the existence of Sxr gene had obtained from the studies of inheritance of a dominant sex-reversal (Sxr) gene. This gene is located on the Y chromosome and it helps the zygotes having two X-chromosomes in the absence of spermatogenesis to develop into males with testes. Actually, these male exhibit X-inactivation and are mosaic for X-linked genes. But by using the recent techniques of DNA manipulation, we can transfer the Sxr containing segment of the Y-chromosomes to the X-chromosome during meiosis. As a consequence, the XX individuals formed develop as males with testes, but the adult XX Sxr males are sterile.
On the basis of research works done on the Sxr trait in mice, it was concluded that any male-sex determining genes carried on the Y-chromosome is essential for regulating the development of a testis from the undifferentiated embryonic gonad. The absence of testis causes the development of an ovary from the undifferentiated gonad.
Tfm Gene: The testes on activation secrete the hormone testosterone, which stimulates the male development. It is regulated by a X-linked gene (Tfm+) specifying a testosterone binding protein present in the cytoplasm of all cells of male and female, which gets activated on binding with testosterone. In the absence of testosterone female development takes place.
Mutations in the Tfm gene causes a syndrome called testicular feminisation including humans, in which instead of Male sexual characteristics, the foetus develops all the external female sexual characteristics. But internally testes develop instead of ovary and the development of fallopian tubes and uterus gets suppressed, which results in blind vagina.
The Sxr and Tfm mutations played an important role in understanding the general features of the process of gonadal differentiation.