Gender differences: the perspective from biology

Abstract

The remarkable thing about sexual differentiation is its diversity. That males are the heterogametic sex, larger than females, more aggressive than females, and the 'non-default' mode of sexual differentiation are concepts not valid throughout most of the animal kingdom. Sex chromosomes are characteristic only of land animals. In birds, the heterogametic sex is female and the sex chromosomes are not related to those of mammals. External factors such as temperature determine sex in lower vertebrates, and there is no similarity among sex-determining genes of different species. The somatic origin of the sex-determining genes and sex chromosomes forces us to ask: what are the other functions of these genes? Because of the obviousness of the sex chromosomes and hormones we may have focused too little on the somatic effects of sex.