Editing Sex-determination system (section)

=== XX/XY sex chromosomes ===
[[File:Drosophila XY sex-determination.svg|thumb|Drosophila sex-chromosomes]]
[[File:Human male karyotpe high resolution - XY chromosome cropped.JPG|thumb|Human male XY chromosomes after [[G-banding]]]]
{{main|XY sex-determination system}}

The '''XX/XY sex-determination system''' is the most familiar, as it is found in humans. The XX/XY system is found in most other [[mammal]]s, as well as some insects. In this system, females have two of the same kind of sex chromosome (XX), while males have two distinct sex chromosomes (XY). The X and Y sex chromosomes are different in shape and size from each other, unlike the rest of the chromosomes ([[autosome]]s), and are sometimes called [[allosome]]s. In some species, such as humans, organisms remain sex indifferent for a time during development ([[embryogenesis]]); in others, however, such as fruit flies, sexual differentiation occurs as soon as the egg is fertilized.<ref name="Hake-2008"/>

==== Y-centered sex determination ====

Some species (including humans) have a gene [[SRY]] on the Y chromosome that determines [[male]]ness. Members of SRY-reliant species can have uncommon XY chromosomal combinations such as [[Klinefelter syndrome|XXY]] and still live.<ref name="Hake-2008">{{cite journal |vauthors=Hake L |title=Genetic Mechanisms of Sex Determination |journal=Nature Education |year=2008 |volume=1 |issue=1 |url=http://www.nature.com/scitable/topicpage/genetic-mechanisms-of-sex-determination-314 |access-date=8 December 2011 |archive-date=19 August 2017 |archive-url=https://web.archive.org/web/20170819121941/http://www.nature.com/scitable/topicpage/genetic-mechanisms-of-sex-determination-314 |url-status=live }}</ref>
Human sex is determined by the presence or absence of a Y chromosome with a functional SRY gene. Once the SRY gene is activated, cells create [[testosterone]] and [[anti-müllerian hormone]] which typically ensures the development of a single, male reproductive system.<ref name="Hake-2008"/> In typical XX embryos, cells secrete [[estrogen]], which drives the body toward the female pathway.

In Y-centered sex determination, the SRY gene is the main gene in determining male characteristics, but multiple genes are required to develop testes. In XY mice, lack of the gene [[DAX1]] on the X chromosome results in sterility, but in humans it causes [[adrenal hypoplasia congenita]].<ref name="Goodfellow-1999">{{cite journal | vauthors = Goodfellow PN, Camerino G | title = DAX-1, an 'antitestis' gene | journal = Cellular and Molecular Life Sciences | volume = 55 | issue = 6–7 | pages = 857–863 | date = June 1999 | pmid = 10412368 | doi = 10.1007/PL00013201 | s2cid = 19764423 | pmc = 11147076 }}</ref> However, when an extra DAX1 gene is placed on the X chromosome, the result is a female, despite the existence of SRY, since it overrides the effects of SRY.<ref name="Chandra-1999">{{cite journal |author=Chandra, H. S. |title=Another way of looking at the enigma of sex determination in Ellobius lutescens |journal=Current Science |date=25 April 1999 |page=1072 |volume=76 |issue=8}}</ref> Even when there are normal sex chromosomes in XX females, duplication or expression of [[SOX9]] causes testes to develop.<ref name="Cox-2011">{{cite journal | vauthors = Cox JJ, Willatt L, Homfray T, Woods CG | title = A SOX9 duplication and familial 46,XX developmental testicular disorder | journal = The New England Journal of Medicine | volume = 364 | issue = 1 | pages = 91–93 | date = January 2011 | pmid = 21208124 | doi = 10.1056/NEJMc1010311 | doi-access = free }}</ref><ref name="Huang-1999">{{cite journal | vauthors = Huang B, Wang S, Ning Y, Lamb AN, Bartley J | title = Autosomal XX sex reversal caused by duplication of SOX9 | journal = American Journal of Medical Genetics | volume = 87 | issue = 4 | pages = 349–353 | date = December 1999 | pmid = 10588843 | doi = 10.1002/(SICI)1096-8628(19991203)87:4<349::AID-AJMG13>3.0.CO;2-N }}</ref> Gradual [[sex reversal]] in developed mice can also occur when the gene [[FOXL2]] is removed from females.<ref name="Uhlenhaut-2009">{{cite journal | vauthors = Uhlenhaut NH, Jakob S, Anlag K, Eisenberger T, Sekido R, Kress J, Treier AC, Klugmann C, Klasen C, Holter NI, Riethmacher D, Schütz G, Cooney AJ, Lovell-Badge R, Treier M | display-authors = 6 | title = Somatic sex reprogramming of adult ovaries to testes by FOXL2 ablation | journal = Cell | volume = 139 | issue = 6 | pages = 1130–1142 | date = December 2009 | pmid = 20005806 | doi = 10.1016/j.cell.2009.11.021 | doi-access = free }}</ref> Even though the gene [[DMRT1]] is used by birds as their sex locus, species who have XY chromosomes also rely upon DMRT1, contained on chromosome 9, for sexual differentiation at some point in their formation.<ref name="Hake-2008"/>

==== X-centered sex determination ====

Some species, such as [[Drosophila melanogaster|fruit flies]], use the presence of two X chromosomes to determine [[female]]ness.<ref name="Penalva-2003">{{cite journal | vauthors = Penalva LO, Sánchez L | title = RNA binding protein sex-lethal (Sxl) and control of Drosophila sex determination and dosage compensation | journal = Microbiology and Molecular Biology Reviews | volume = 67 | issue = 3 | pages = 343–59, table of contents | date = September 2003 | pmid = 12966139 | pmc = 193869 | doi = 10.1128/MMBR.67.3.343-359.2003 }}</ref> Species that use the number of Xs to determine sex are nonviable with an extra X chromosome.

==== 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]]