Editing Sexual differentiation (section)

==Sex determination systems==
{{Main|Sex determination system}}Humans, many [[Mammal|mammals]], and some [[Insect|insects]] and other animals have an [[XY sex-determination system]]. Humans have 46 [[Chromosome|chromosomes]], including two [[Sex chromosome|sex chromosomes]], XX in females and XY in males. The [[Y chromosome]] must carry at least one [[essential gene]] which determines [[testis|testicular]] formation (originally termed ''TDF'').<ref name=":1">{{Cite journal|last1=Wilhelm|first1=Dagmar|last2=Palmer|first2=Stephen|last3=Koopman|first3=Peter|date=2007-01-01|title=Sex Determination and Gonadal Development in Mammals|journal=Physiological Reviews|volume=87|issue=1|pages=1–28|doi=10.1152/physrev.00009.2006|pmid=17237341|issn=0031-9333}}</ref> In [[Transgene|transgenic]] XX mice (and some human [[XX male syndrome|XX male]]s), the [[sex-determining region Y protein]] alone is sufficient to induce male differentiation.<ref>{{Cite journal|last=Gilbert|first=Scott F.|date=2000|title=Chromosomal Sex Determination in Mammals|url=https://www.ncbi.nlm.nih.gov/books/NBK9967/|journal=Developmental Biology. 6th Edition|language=en}}</ref>

Other chromosomal systems exist in different [[taxa]], such as the [[ZW sex-determination system]] in birds<ref name=":22">{{Cite journal|last1=Chue|first1=J|last2=Smith|first2=C|date=2011-01-31|title=Sex Determination and Sexual Differentiation in the Avian Model|journal=The FEBS Journal|volume=278|issue=7|pages=1027–34|doi=10.1111/j.1742-4658.2011.08032.x|language=en|pmid=21281451|s2cid=24751510}}</ref> and the [[X0 sex-determination system|XO system]] in insects.<ref>{{Cite journal|last1=Blackmon|first1=Heath|last2=Ross|first2=Laura|last3=Bachtrog|first3=Doris|date=January 2017|title=Sex Determination, Sex Chromosomes, and Karyotype Evolution in Insects|journal=Journal of Heredity|volume=108|issue=1|pages=78–93|doi=10.1093/jhered/esw047|issn=0022-1503|pmc=6281344|pmid=27543823}}</ref>

[[Environmental sex determination]] refers to the determination (and then differentiation) of sex via non-genetic cues like social factors, temperature, and available nutrients. In some species, such as [[clownfish]] (known to be universally [[Hermaphrodite|hermaphroditic]]), sex differentiation can occur more than once as a response to different environmental cues,<ref name=":13">{{Cite journal|last1=Casas|first1=Laura|last2=Saborido-Rey|first2=Fran|last3=Ryu|first3=Taewoo|last4=Michell|first4=Craig|last5=Ravasi|first5=Timothy|last6=Irigoien|first6=Xabier|date=2016-10-17|title=Sex Change in Clownfish: Molecular Insights from Transcriptome Analysis|journal=Scientific Reports|volume=6|pages=35461|doi=10.1038/srep35461|issn=2045-2322|pmc=5066260|pmid=27748421|bibcode=2016NatSR...635461C}}</ref> offering an example of how sex differentiation does not always follow a linear path.

There have been multiple transitions between environmental and genetic sex determination systems in reptiles over time,<ref name=":02">{{Cite journal|last1=Rhen|first1=T.|last2=Schroeder|first2=A.|date=March 2010|title=Molecular Mechanisms of Sex Determination in Reptiles|journal=Sexual Development|volume=4|issue=1–2|pages=16–28|doi=10.1159/000282495|issn=1661-5425|pmc=2918650|pmid=20145384}}</ref> and recent studies have shown that temperature can sometimes override sex determination via chromosomes.<ref>{{Cite journal|last1=Pokorná|first1=Martina|last2=Kratochvíl|first2=Lukáš|date=2009-05-01|title=Phylogeny of sex-determining mechanisms in squamate reptiles: are sex chromosomes an evolutionary trap?|journal=Zoological Journal of the Linnean Society|language=en|volume=156|issue=1|pages=168–183|doi=10.1111/j.1096-3642.2008.00481.x|issn=0024-4082|doi-access=free}}</ref>