sexdetermination

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 the development of the gonads and the production of sex hormones.

Genetic sex determination uses chromosomes to set the default developmental pathway. In many animals, the combination of sex chromosomes determines sex: XX or XY in mammals, ZZ or ZW in birds, and various other systems such as XX-XY, XO, or haplodiploidy in different groups. In therian mammals, the presence of a functioning SRY gene on the Y chromosome typically triggers male development, leading to the formation of testes; in its absence, ovaries develop. Development then proceeds through hormones that shape the internal reproductive tract and secondary characteristics. Not all species rely on a single, rigid chromosome system; some have sex-determining genes that are not SRY-like, and chromosomal anomalies can produce intersex individuals.

Environmental sex determination allocates sex based on external conditions. Temperature-dependent sex determination (TSD) is common in reptiles: the ambient temperature during a sensitive period biases the sex ratio toward one sex. The pattern of bias is species-specific, and timings and durations of exposure matter. In some fish and invertebrates, social context or environmental cues can also influence or determine sex, and some species exhibit sequential hermaphroditism, changing sex in response to life-history conditions.

Understanding sex determination sheds light on development, ecology, and evolution, and has medical relevance for disorders of sex development and population management.