XO sex-determination system

This system determines the sex of offspring among: 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 (XO).

[original research] In humans the XO designation attaches to individuals with Turner syndrome.

[clarification needed] It typically evolves due to Y-chromosome degeneration.

As the Y-chromosome is not paired (though see pseudoautosomal region), it is susceptible to decay by Muller's ratchet.

Parthenogenesis with XO sex-determination can occur by different mechanisms to produce either male or female offspring.