X0 sex-determination system
The X0 sex-determination system is a system that determines the sex of offspring among:
- most arachnids with the exception of mites where a small majority are haplodiploid,
- almost all apterygote and Paleopteran insects (e.g. dragonflies and silverfish)
- most exopterygote insects, (e.g. grasshoppers, crickets and cockroaches)
- some nematodes, crustaceans, gastropod molluscs and bony fish, notably in the genus Ancistrus.
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). The zero (sometimes, the letter O) signifies the lack of a second X. Maternal gametes always contain an X chromosome, so the sex of the animals’ offspring depends on whether a sex chromosome is present in the male gamete. Its sperm normally contain either one X chromosome or no sex chromosomes at all.
In a variant of this system, most individuals have two sex chromosomes (XX) and are hermaphroditic, producing both eggs and sperm with which they can fertilize themselves, while rare individuals are male and have only one sex chromosome (X0). The model organism Caenorhabditis elegans—a nematode frequently used in biological research—is one such organism.
In human females, having only a single X chromosome is known as Turner syndrome.
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