Integrative Topics in Physiology

Abstract

The presence of the Y chromosome in the male dictates the development of a testis; the hormonal secretions of the testis then impose male development on the phenotypically indifferent fetus. In the absence of a Y chromosome a testis fails to develop and a female phenotype results. Thus, a central concept in sexual differentiation is that the male is the induced phenotype, whereas the female develops passively in the absence of male determinants. The genotype of the female is 46,XX and that of the male is 46,XY. All oocytes have a 23,X complement, and spermatozoa have either 23,X or 23,Y. The presence of a single Y chromosome ensures testicular differentiation and the development of a predominantly male phenotype.