Mechanisms of genetic sex determination, gonadal sex differentiation, and germ-cell development in animals

This chapter focuses on the basic events of genetic sex determination and primary sex differentiation. Several models are discussed to explain the genetics of sex determination. The most recent and best developed theory of sex determination based on genetic inheritance involves expression of the gene for the H-Y cell-surface antigen in male cells. Observations in several mammalian species have revealed a consistent correlation between the presence of H-Y antigen and testicular development. Sexual differentiation of the gonad results from the development of only one gonadal component, either the cortex or the medulla. Differentiation of the testis is marked by continued development of the medullary cords, into which the cortical germ cells migrate. There are certain important distinctions between the mammalian and avian gonadal development. The most striking difference is that in the avian female only the left gonad differentiates into a functional ovary, whereas in mammals female gonad development is bilaterally symmetrical.