Editing Sex-determination system (section)

==== Other variants of XX/XY sex determination ====

Some fish have variants of the [[XY sex-determination system]], as well as the regular system. For example, while having an XY format, ''[[Xiphophorus nezahualcoyotl]]'' and ''X. milleri'' also have a second Y chromosome, known as Y', that creates XY' females and YY' males.<ref name="Schartl-2004a">{{cite journal | vauthors = Schartl M | title = A comparative view on sex determination in medaka | journal = Mechanisms of Development | volume = 121 | issue = 7–8 | pages = 639–645 | date = July 2004 | pmid = 15210173 | doi = 10.1016/j.mod.2004.03.001 | s2cid = 17401686 | doi-access = free }}</ref>

At least one [[monotreme]], the [[platypus#Evolution|platypus]], presents a particular sex determination scheme that in some ways resembles that of the [[ZW sex-determination system|ZW sex chromosomes]] of birds and lacks the SRY gene. The [[platypus]] has sex chromosomes <math>X_1, X_2, X_3, X_4, X_5, Y_1, Y_2, Y_3, Y_4, Y_5</math>. The males have <math>X_1Y_1/X_2Y_2/X_3Y_3/X_4Y_4/X_5Y_5</math>, while females have <math>X_1X_1/X_2X_2/X_3X_3/X_4X_4/X_5X_5</math>. During meiosis, 5 of X form one chain, and 5 of Y form another chain. Thus, they behave effectively as a typical XY chromosomal system, except each of X and Y is broken into 5 parts, with the effect that [[Homologous recombination|recombinations]] occur very frequently at 4 particular points.<ref>{{Cite journal |last1=Gruetzner |first1=Frank |last2=Ashley |first2=Terry |last3=Rowell |first3=David M. |last4=Marshall Graves |first4=Jennifer A. |date=2006-04-01 |title=How did the platypus get its sex chromosome chain? A comparison of meiotic multiples and sex chromosomes in plants and animals |url=https://doi.org/10.1007/s00412-005-0034-4 |journal=Chromosoma |volume=115 |issue=2 |pages=75–88 |doi=10.1007/s00412-005-0034-4 |pmid=16344965 |issn=1432-0886}}</ref> One of the X chromosomes is homologous to the human X chromosome, and another is homologous to the bird Z chromosome.<ref>{{Cite journal |last1=Grützner |first1=Frank |last2=Rens |first2=Willem |last3=Tsend-Ayush |first3=Enkhjargal |last4=El-Mogharbel |first4=Nisrine |last5=O'Brien |first5=Patricia C. M. |last6=Jones |first6=Russell C. |last7=Ferguson-Smith |first7=Malcolm A. |last8=Marshall Graves |first8=Jennifer A. |date=December 2004 |title=In the platypus a meiotic chain of ten sex chromosomes shares genes with the bird Z and mammal X chromosomes |url=https://www.nature.com/articles/nature03021 |journal=Nature |volume=432 |issue=7019 |pages=913–917 |doi=10.1038/nature03021 |pmid=15502814 |bibcode=2004Natur.432..913G |issn=1476-4687}}</ref>

Although it is an XY system, the platypus' sex chromosomes share no homologues with [[eutherian]] sex chromosomes.<ref name="Warren-2008">{{cite journal | vauthors = Warren WC, Hillier LW, Marshall Graves JA, Birney E, Ponting CP, Grützner F, Belov K, Miller W, Clarke L, Chinwalla AT, Yang SP, Heger A, Locke DP, Miethke P, Waters PD, Veyrunes F, Fulton L, Fulton B, Graves T, Wallis J, Puente XS, López-Otín C, Ordóñez GR, Eichler EE, Chen L, Cheng Z, Deakin JE, Alsop A, Thompson K, Kirby P, Papenfuss AT, Wakefield MJ, Olender T, Lancet D, Huttley GA, Smit AF, Pask A, Temple-Smith P, Batzer MA, Walker JA, Konkel MK, Harris RS, Whittington CM, Wong ES, Gemmell NJ, Buschiazzo E, Vargas Jentzsch IM, Merkel A, Schmitz J, Zemann A, Churakov G, Kriegs JO, Brosius J, Murchison EP, Sachidanandam R, Smith C, Hannon GJ, Tsend-Ayush E, McMillan D, Attenborough R, Rens W, Ferguson-Smith M, Lefèvre CM, Sharp JA, Nicholas KR, Ray DA, Kube M, Reinhardt R, Pringle TH, Taylor J, Jones RC, Nixon B, Dacheux JL, Niwa H, Sekita Y, Huang X, Stark A, Kheradpour P, Kellis M, Flicek P, Chen Y, Webber C, Hardison R, Nelson J, Hallsworth-Pepin K, Delehaunty K, Markovic C, Minx P, Feng Y, Kremitzki C, Mitreva M, Glasscock J, Wylie T, Wohldmann P, Thiru P, Nhan MN, Pohl CS, Smith SM, Hou S, Nefedov M, de Jong PJ, Renfree MB, Mardis ER, Wilson RK | display-authors = 6 | title = Genome analysis of the platypus reveals unique signatures of evolution | journal = Nature | volume = 453 | issue = 7192 | pages = 175–183 | date = May 2008 | pmid = 18464734 | pmc = 2803040 | doi = 10.1038/nature06936 | bibcode = 2008Natur.453..175W }}</ref> Instead, homologues with eutherian sex chromosomes lie on the platypus chromosome 6, which means that the eutherian sex chromosomes were [[autosomes]] at the time that the monotremes diverged from the therian mammals (marsupials and eutherian mammals). However, homologues to the avian [[DMRT1]] gene on platypus sex chromosomes X3 and X5 suggest that it is possible the sex-determining gene for the platypus is the same one that is involved in bird sex-determination. More research must be conducted in order to determine the exact sex determining gene of the platypus.<ref name="Gruetzner-2006">{{cite journal | vauthors = Gruetzner F, Ashley T, Rowell DM, Marshall Graves JA | title = How did the platypus get its sex chromosome chain? A comparison of meiotic multiples and sex chromosomes in plants and animals | journal = Chromosoma | volume = 115 | issue = 2 | pages = 75–88 | date = April 2006 | pmid = 16344965 | doi = 10.1007/s00412-005-0034-4 | name-list-style = amp | s2cid = 23603889 }}</ref>

[[File:Critique of the Theory of Evolution Fig 060.svg |thumb|Heredity of sex chromosomes in XO sex determination]]