A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most sexual organisms have two sexes. Occasionally there are hermaphrodites in place of one or both sexes. There are also some species that are only one sex due to parthenogenesis, the act of a female reproducing without fertilization. In many cases, sex determination is genetic: males and females have different alleles or even different genes that specify their sexual morphology. In animals, this is often accompanied by chromosomal differences. Determination genetically is generally through chromosome combinations of XY, ZW, XO, ZO, or haplodiploid. Sexual differentiation is generally started by a main gene, a sex locus, then a multitude of other genes follow in a domino effect. In other cases, sex is determined by environmental variables (such as temperature) or social variables (the size of an organism relative to other members of its population). Environmental sex determination occurred before genetic; it is thought that a temperature-dependent reptile was the common ancestor to sex chromosomes. Some species do not have a set sex, and instead change it based on certain cues. The details of some sex-determination systems are not yet fully understood.
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