sex-determination system X0 sex-determination system https://en.wikipedia.org/wiki/X0_sex-determination_system https://www.wikidata.org/wiki/Q607251 A system that determines the sex of offspring among most arachnids, almost all apterygote and paleopteran insects, most exoptergote insects, and some nematodes, crustaceans, gastropod molluscs, and bony fish. In this system, there is only one sex chromosome, referred to as X. Males only have one X chromosome (X0), while females have two (XX).