Editing Sex-determination system (section)

=== ZW/ZZ sex chromosomes ===
{{Main |ZW sex-determination system}}

The '''ZW sex-determination system''' is found in birds, some reptiles, and some insects and other organisms. The ZW sex-determination system is reversed compared to the XY system: females have two different kinds of [[chromosomes]] (ZW), and males have two of the same kind of chromosomes (ZZ). In the chicken, this was found to be dependent on the expression of DMRT1.<ref name="Smith-2009">{{cite journal | vauthors = Smith CA, Roeszler KN, Ohnesorg T, Cummins DM, Farlie PG, Doran TJ, Sinclair AH | title = The avian Z-linked gene DMRT1 is required for male sex determination in the chicken | journal = Nature | volume = 461 | issue = 7261 | pages = 267–271 | date = September 2009 | pmid = 19710650 | doi = 10.1038/nature08298 | s2cid = 4413389 | bibcode = 2009Natur.461..267S }}</ref> In birds, the genes FET1 and ASW are found on the W chromosome for females, similar to how the Y chromosome contains SRY.<ref name="Hake-2008"/> However, not all species depend upon the W for their sex. For example, there are moths and butterflies that are ZW, but some have been found female with ZO, as well as female with ZZW.<ref name="Majerus-2003"/> Also, while mammals deactivate one of their extra X chromosomes when female, it appears that in the case of [[Lepidoptera]], the males produce double the normal amount of enzymes, due to having two Z's.<ref name="Majerus-2003"/> Because the use of ZW sex determination is varied, it is still unknown how exactly most species determine their sex.<ref name="Majerus-2003"/> However, reportedly, the silkworm ''Bombyx mori'' uses a single female-specific [[Piwi-interacting RNA|piRNA]] as the primary determiner of sex.<ref>{{cite journal | vauthors = Kiuchi T, Koga H, Kawamoto M, Shoji K, Sakai H, Arai Y, Ishihara G, Kawaoka S, Sugano S, Shimada T, Suzuki Y, Suzuki MG, Katsuma S | display-authors = 6 | title = A single female-specific piRNA is the primary determiner of sex in the silkworm | journal = Nature | volume = 509 | issue = 7502 | pages = 633–636 | date = May 2014 | pmid = 24828047 | doi = 10.1038/nature13315 | s2cid = 205238635 | bibcode = 2014Natur.509..633K }}</ref> Despite the similarities between the ZW and XY systems, these sex chromosomes evolved separately. In the case of the chicken, their Z chromosome is more similar to humans' autosome 9.<ref name="Stiglec-2007">{{cite journal | vauthors = Stiglec R, Ezaz T, Graves JA | title = A new look at the evolution of avian sex chromosomes | journal = Cytogenetic and Genome Research | volume = 117 | issue = 1–4 | pages = 103–109 | year = 2007 | pmid = 17675850 | doi = 10.1159/000103170 | s2cid = 12932564 }}</ref> The chicken's Z chromosome also seems to be related to the X chromosome of the platypus.<ref name="Grützner-2004">{{cite journal | vauthors = Grützner F, Rens W, Tsend-Ayush E, El-Mogharbel N, O'Brien PC, Jones RC, Ferguson-Smith MA, Marshall Graves JA | display-authors = 6 | title = In the platypus a meiotic chain of ten sex chromosomes shares genes with the bird Z and mammal X chromosomes | journal = Nature | volume = 432 | issue = 7019 | pages = 913–917 | date = December 2004 | pmid = 15502814 | doi = 10.1038/nature03021 | name-list-style = amp | s2cid = 4379897 | bibcode = 2004Natur.432..913G }}</ref> When a ZW species, such as the [[Komodo dragon]], reproduces [[Parthenogenesis|parthenogenetically]], usually only males are produced. This is due to the fact that the haploid eggs double their chromosomes, resulting in ZZ or WW. The ZZ become males, but the WW are not viable and are not brought to term.<ref name="BBCNews-2006">{{cite news |work=BBC News |url=http://news.bbc.co.uk/2/hi/science/nature/6196225.stm |title=Virgin births for giant lizards |access-date=13 March 2008 |date=20 December 2006 |archive-date=4 November 2014 |archive-url=https://web.archive.org/web/20141104002358/http://news.bbc.co.uk/2/hi/science/nature/6196225.stm |url-status=live }}</ref>

In both XY and ZW sex determination systems, the sex chromosome carrying the critical factors is often significantly smaller, carrying little more than the genes necessary for triggering the development of a given sex.<ref>{{cite web|title=Evolution of the Y Chromosome|url=http://www.learner.org/channel/courses/biology/textbook/gender/gender_4.html|url-status=dead|archive-url=https://web.archive.org/web/20041104181945/http://www.learner.org/channel/courses/biology/textbook/gender/gender_4.html|archive-date=November 4, 2004|access-date=1 April 2008|website=Annenberg Media}}</ref>{{Better source needed|date=May 2021}}