Editing Sex-determination system (section)

=== Homomorphism and the fountain of youth ===
It is an evolutionary puzzle why certain sex chromosomes remain homomorphic over millions of years, especially among lineages of fishes, amphibians, and nonavian reptiles. The fountain-of-youth model states that heteromorphy results from recombination suppression, and recombination suppression results from the male phenotype, not the sex chromosomes themselves. Therefore, if some XY sex-reversed females are fertile and adaptive under some circumstances, then the X and Y chromosomes would recombine in these individuals, preventing Y chromosome decay and maintaining long-term homomorphism.<ref>{{Cite journal |last=Perrin |first=Nicolas |date=December 2009 |title=Sex Reversal: A Fountain of Youth for Sex Chromosomes? |url=https://academic.oup.com/evolut/article/63/12/3043/6881025 |journal=Evolution |volume=63 |issue=12 |pages=3043–3049 |doi=10.1111/j.1558-5646.2009.00837.x|pmid=19744117 }}</ref>

[[Sex reversal]] denotes a situation where the phenotypic sex is different from the genotypic sex. While in humans, [[Disorders of sex development|sex reversal]] (such as the [[XX male syndrome]]) are often infertile, sex-reversed individuals of some species are fertile under some conditions. For example, some XY-individuals in population of [[Chinook salmon]] in the [[Columbia River]] became fertile females, producing YY sons. Since Chinook salmons have homomorphic sex chromosomes, such YY sons are healthy. When YY males mate with XX females, all their progeny would be XY male if grown under normal conditions.<ref>{{Cite journal |last1=Nagler |first1=J J |last2=Bouma |first2=J |last3=Thorgaard |first3=G H |last4=Dauble |first4=D D |date=January 2001 |title=High incidence of a male-specific genetic marker in phenotypic female chinook salmon from the Columbia River. |journal=Environmental Health Perspectives |volume=109 |issue=1 |pages=67–69 |doi=10.1289/ehp.0110967 |issn=0091-6765 |pmc=1242053 |pmid=11171527}}</ref>

Support for the hypothesis is found in the [[common frog]], for which XX males and XY males both suppresses sex chromosome recombination, but XX and XY females both recombine at the same rate.<ref>{{Cite journal |last1=Rodrigues |first1=Nicolas |last2=Studer |first2=Tania |last3=Dufresnes |first3=Christophe |last4=Perrin |first4=Nicolas |date=2018-04-01 |title=Sex-Chromosome Recombination in Common Frogs Brings Water to the Fountain-of-Youth |url=https://academic.oup.com/mbe/article/35/4/942/4823133 |journal=Molecular Biology and Evolution |volume=35 |issue=4 |pages=942–948 |doi=10.1093/molbev/msy008 |issn=0737-4038}}</ref>