The Genetic and Endocrinological Basis for Sex Differentiation

Abstract

The reasons why sexual reproduction replaced asexual reproduction during the evolution of higher organisms is a basic problem of biology that has never been resolved (Williams 1975) (1). It is also true that little is known about the molecular biology by which the two sexes develop. It is true, of course, that the chromosomal basis of sex determination has been established for many years and that in higher mammals the heterogametic sex in which the two sex chromosomes are different is male and the homogametic sex is female. Nevertheless, the fact that the ultimate control of sex is chromosomal or genetic does not explain the mechanisms by which sexual differentiation takes place during embryogenesis. If sex is determined at the moment of conception one would logically expect that the two embryos would develop differently from the outset and that male and female embryos would be distinguishable early in development. However, during the initial weeks of gestation sexual differentiation is indifferent. It is not possible during this phase of embryogenesis to distinguish male and female embryos except by performing chromosomal analyses; in other regards embryos of both sexes are identical. However, starting about the ninth week of gestation gonadal differentiation takes place, namely, an indifferent gonad changes into testis or ovary, and then the sexual phenotype becomes clearly differentiated thereafter.

The original work described in this paper was supported by grant AM03892 from the U. S. Public Health Service.