sexdetermining

Sex determination is the biological process by which an organism develops as male, female, or another sex. It can be genetic, environmental, or a combination of factors, and it often interacts with later processes of sex differentiation to shape reproductive anatomy and physiology.

Genetic sex determination (GSD) uses chromosome composition to set an organism’s developmental trajectory. The best-known systems are XY, in which males are typically XY and females XX, found in most mammals, and ZZ/ZW, in which males are ZZ and females ZW, observed in birds and some reptiles. In many XY species, the presence of a Y chromosome bearing the SRY gene triggers a cascade that promotes testis formation, producing male hormones and guiding male development. In ZZ/ZW species, different master genes establish the undifferentiated gonads as ovaries or testes, and different dosage effects (such as DMRT1) can influence outcomes. Some organisms exhibit haplodiploidy, where fertilized eggs become females and unfertilized eggs become males, showing a different genetic mechanism for sex determination.

Environmental sex determination (ESD) occurs when external cues dictate sex. Temperature-dependent sex determination is common in reptiles, where incubation temperature biases the offspring’s sex. Other environmental factors, including social structure and population density, can also influence sex in various species, especially among fish and invertebrates.

Some taxa display flexible or sequential sex determination, such as sequential hermaphroditism in many fishes, where an individual changes sex in response to social or environmental conditions. Across taxa, sex determination systems vary widely and can evolve rapidly, reflecting adaptations to ecological and life-history traits.