XO sex-determination system

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Heredity of sex chromosomes in XO sex determination

The XO sex-determination system (sometimes X0 sex-determination system) is a system that some species of insects, arachnids, and mammals use to determine the sex of offspring. In this system, there is only one sex chromosome, referred to as X. Males only have one X chromosome (XO), while females have two (XX). The letter O (sometimes a zero) 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 contains either one X chromosome or no sex chromosomes at all.

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). The model organism Caenorhabditis elegans—a nematode frequently used in biological research—is one such organism.

Some Drosophila species have XO males.[9] These are thought to arise via the loss of the Y chromosome.[original research]


XO sex determination can evolve from XY sex determination with about 2 million years.[10]


Parthenogenesis with XO sex-determination can occur by different mechanisms to produce either male or female offspring.[11]

See also[edit]


  1. ^ a b c Bull, James J.; Evolution of sex determining mechanisms; p. 17 ISBN 0805304002
  2. ^ Bachtrog, Doris; Mank, Judith E.; Peichel, Catherine L.; Kirkpatrick, Mark; Otto, Sarah P.; Ashman, Tia-Lynn; Hahn, Matthew W.; Kitano, Jun; Mayrose, Itay; Ming, Ray; Perrin, Nicolas; Ross, Laura; Valenzuela, Nicole; Vamosi, Jana C. and The Tree of Sex Consortium; ‘Sex Determination: Why So Many Ways of Doing It?’; PLoS Biol12(7): e1001899
  3. ^ Thirot-Quiévreux, Catherine (2003). "Advances in Chromosomal Studies of Gastropod Molluscs". Journal of Molluscan Studies. 69 (3): 187–201. doi:10.1093/mollus/69.3.187.
  4. ^ Devlin, R.H. and Y. Nagahama, 2002. ‘Sex determination and sex differentiation in fish: an overview of genetic, physiological, and environmental influences’; Aquaculture 208: 191–364.
  5. ^ Anderson, Luís Alves; Oliveira, Claudio; Nirchio, Mauro; Granado, Ángel and Foresti, Fausto; ‘Karyotypic relationships among the tribes of Hypostominae (Siluriformes: Loricariidae) with description of XO sex chromosome system in a Neotropical fish species’; Genetica, vol. 128 (2006); pp. 1-9
  6. ^ Hsu, T. C.; Benirschke, Kurt (1977). "Hypsignathus monstrosus (Hammer-headed fruit bat)". An Atlas of Mammalian Chromosomes. pp. 13–16. doi:10.1007/978-1-4615-6436-2_4. ISBN 978-1-4684-7997-3.
  7. ^ Denys, C.; Kadjo, B.; Missoup, A. D.; Monadjem, A.; Aniskine, V. (2013). "New records of bats (Mammalia: Chiroptera) and karyotypes from Guinean Mount Nimba (West Africa)". Italian Journal of Zoology. 80 (2): 279–290. doi:10.1080/11250003.2013.775367. hdl:2263/42399. S2CID 55842692.
  8. ^ Kobayashi, Tsuyoshi; Yamada, Fumio; Hashimoto, Takuma; Abe, Shintaro; Matsuda, Yoichi; Kuroiwa, Asato (2007). "Exceptional minute sex-specific region in the XO mammal, Ryukyu spiny rat". Chromosome Research. 15 (2): 175–187. doi:10.1007/s10577-006-1093-y. PMID 17294259. S2CID 6461447.
  9. ^ Patterson, J. T.; Stone, W. S. (1952). Evolution in the Genus Drosophila. New York: Macmillan.
  10. ^ Nei, Masatoshi (2013-05-02). Mutation-Driven Evolution. OUP Oxford. p. 168. ISBN 978-0-19-163781-0.
  11. ^ Hales, Dinah F.; Alex C. C. Wilson; Mathew A. Sloane; Jean-Christophe Simon; Jean-François Legallic; Paul Sunnucks (2002). "Lack of Detectable Genetic Recombination on the X Chromosome During the Parthenogenetic Production of Female and Male Aphids". Genetics Research. 79 (3): 203–209. doi:10.1017/S0016672302005657. PMID 12220127.