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The X0 sex-determination system is a system that determines the sex of offspring among grasshoppers, crickets, cockroaches, and some other insects. 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.
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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.
Some Drosophila species have X0 males. These are thought to arise via the loss of the Y chromosome.
In humans
In humans, having only a single X chromosome is known as Turner syndrome. However, such individuals generally present as female, as humans have an XY sex-determination system.
Parthenogenesis
Parthenogenesis with X0 sex-determination can occur by different mechanisms to produce either male or female offspring.