Sex-determining mechanisms in animals.

Biological mechanisms leading to the development of males and females are extremely varied. In the XX/XY system, the male has an unequal pair of chromosomes, while in the ZZ/ZW system, the unequal pair is in the female. Sex can also be determined by the temperature of incubation. Recent research has focused on the identification of sex-determining genes, culminating in the demonstration that the Sry gene on the Y chromosome of mice can induce male development in genetically female XX mouse embryos. Nevertheless, the occurrence of phenotypes in apparent contrast to the genotype suggests that the genetic male/female switch is not simple, and there may be common features linking different sex-determining mechanisms. There is increasing evidence that such a link may be provided by the induction of growth differences, and that the primary sex difference may result from the distinction between fast versus slow growth.