Editing Sex-determination system (section)

==Chromosomal systems==
Among animals, the most common chromosomal sex determination systems are XY, XO, ZW, ZO, but with numerous exceptions.

According to the Tree of Sex database<ref>{{Cite journal |date=2014-06-24 |title=Tree of Sex: A database of sexual systems |journal=Scientific Data |volume=1 |pages=140015 |doi=10.1038/sdata.2014.15 |issn=2052-4463 |pmc=4322564 |pmid=25977773 |last1=Ashman |first1=Tia-Lynn |last2=Bachtrog |first2=Doris |last3=Blackmon |first3=Heath |last4=Goldberg |first4=Emma E. |last5=Hahn |first5=Matthew W. |last6=Kirkpatrick |first6=Mark |last7=Kitano |first7=Jun |last8=Mank |first8=Judith E. |last9=Mayrose |first9=Itay |last10=Ming |first10=Ray |last11=Otto |first11=Sarah P. |last12=Peichel |first12=Catherine L. |last13=Pennell |first13=Matthew W. |last14=Perrin |first14=Nicolas |last15=Ross |first15=Laura |last16=Valenzuela |first16=Nicole |last17=Vamosi |first17=Jana C. }}</ref> (as of 2023), the known sex determination systems are:<ref>{{Cite journal |last1=Hayashi |first1=Shun |last2=Abe |first2=Takuya |last3=Igawa |first3=Takeshi |last4=Katsura |first4=Yukako |last5=Kazama |first5=Yusuke |last6=Nozawa |first6=Masafumi |date=2024-07-31 |title=Sex chromosome cycle as a mechanism of stable sex determination |url=https://academic.oup.com/jb/article/176/2/81/7709515 |journal=The Journal of Biochemistry |volume=176 |issue=2 |pages=81–95 |doi=10.1093/jb/mvae045 |issn=0021-924X |pmc=11289310 |pmid=38982631}}</ref>
{| class="wikitable"
|+ Sex determination systems in vertebrates, insects and angiosperms
! Taxonomic group !! XY !! XO !! ZW !! ZO !! Other<sup>1</sup> !! XO/XY ratio !! ZO/ZW ratio
|-
| Vertebrates || 722 || 15 || 480 || 3 || 254 || 0.02 || 0.01 
|-
| Insects || 4415 || 1857 || 37 || 25 || 156 || 0.42 || 0.68
|-
| Angiosperms || 23 || 0 || 1 || 0 || 19 || 0.00 || 0.00
|-
| Total || 5160 || 1872 || 518 || 28 || 429 || 0.36 || 0.05
|}

<sup>1. complex sex chromosomes, homomorphic sex chromosomes, or others</sup>

=== 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]]

=== XX/X0 sex chromosomes ===
{{Main |X0 sex-determination system}}

In this variant of the XY system, females have two copies of the sex chromosome (XX) but males have only one (X0). The ''0'' denotes the absence of a second sex chromosome. Generally in this method, the sex is determined by amount of genes expressed across the two chromosomes. This system is observed in a number of insects, including the grasshoppers and crickets of order [[Orthoptera]] and in cockroaches (order [[cockroach|Blattodea]]). A small number of mammals also lack a Y chromosome. These include the Amami spiny rat (''[[Tokudaia osimensis]]'') and the Tokunoshima spiny rat (''[[Tokudaia tokunoshimensis]]'') and ''Sorex araneus'', a [[shrew]] species. Transcaucasian mole voles (''[[Ellobius lutescens]]'') also have a form of XO determination, in which both sexes lack a second sex chromosome.<ref name="Chandra-1999"/> The mechanism of sex determination is not yet understood.<ref name="Kuroiwa-2011">{{cite journal | vauthors = Kuroiwa A, Handa S, Nishiyama C, Chiba E, Yamada F, Abe S, Matsuda Y | title = Additional copies of CBX2 in the genomes of males of mammals lacking SRY, the Amami spiny rat (Tokudaia osimensis) and the Tokunoshima spiny rat (Tokudaia tokunoshimensis) | journal = Chromosome Research | volume = 19 | issue = 5 | pages = 635–644 | date = July 2011 | pmid = 21656076 | doi = 10.1007/s10577-011-9223-6 | s2cid = 23311263 }}</ref>

The [[nematode]] ''[[Caenorhabditis elegans|C. elegans]]'' is male with one sex chromosome (X0); with a pair of chromosomes (XX) it is a hermaphrodite.<ref name="Majerus-2003">{{Cite book | vauthors = Majerus ME | title = Sex wars: genes, bacteria, and biased sex ratios | publisher = Princeton University Press | pages = 250 | url = https://books.google.com/books?id=vDHOYPQ2mmYC&q=zo,+zww,+zzww+lepidoptera | isbn = 978-0-691-00981-0 | access-date = 4 November 2011 | year = 2003 }}
</ref> Its main sex gene is XOL, which encodes [[XOL-1 Switch protein N-terminal domain|XOL-1]] and also controls the expression of the genes TRA-2 and HER-1. These genes reduce male gene activation and increase it, respectively.<ref name="Kuwabara-1992">{{cite journal | vauthors = Kuwabara PE, Okkema PG, Kimble J | title = tra-2 encodes a membrane protein and may mediate cell communication in the Caenorhabditis elegans sex determination pathway | journal = Molecular Biology of the Cell | volume = 3 | issue = 4 | pages = 461–473 | date = April 1992 | pmid = 1498366 | pmc = 275596 | doi = 10.1091/mbc.3.4.461 }}</ref>

=== 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}}

=== ZZ/Z0 sex chromosomes ===
{{Main |Z0 sex-determination system}}

The '''ZZ/Z0 sex-determination system''' is found in some moths. In these insects there is one sex chromosome, Z. Males have two Z chromosomes, whereas females have one Z. Males are ZZ, while females are Z0.<ref name="Traut-2007">{{cite journal | vauthors = Traut W, Sahara K, Marec F | title = Sex chromosomes and sex determination in Lepidoptera | journal = Sexual Development | volume = 1 | issue = 6 | pages = 332–346 | date = 2007 | pmid = 18391545 | doi = 10.1159/000111765 | s2cid = 6885122 }}</ref><ref>{{cite web|url=http://www.nature.com/scitable/topicpage/genetic-mechanisms-of-sex-determination-314|title=Genetic Mechanisms of Sex Determination - Learn Science at Scitable|website=www.nature.com|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><ref>{{cite book|url=https://books.google.com/books?id=5w8FgSGuH34C&q=ZO+sex-determination+system+moth&pg=PA461|title=Handbuch Der Zoologie / Handbook of Zoology|publisher=Walter de Gruyter|via=Google Books|isbn=9783110162103|year=1925}}</ref>

===UV sex chromosomes===
In some [[bryophyte]] and some [[algae]] species, the [[gametophyte]] stage of the life cycle, rather than being hermaphrodite, occurs as separate male or female individuals that produce male and female gametes respectively. When meiosis occurs in the [[sporophyte]] generation of the life cycle, the sex chromosomes known as U and V assort in spores that carry either the U chromosome and give rise to female gametophytes, or the V chromosome and give rise to male gametophytes.<ref name="Bachtrog-2011">{{cite journal | vauthors = Bachtrog D, Kirkpatrick M, Mank JE, McDaniel SF, Pires JC, Rice W, Valenzuela N | title = Are all sex chromosomes created equal? | journal = Trends in Genetics | volume = 27 | issue = 9 | pages = 350–357 | date = September 2011 | pmid = 21962970 | doi = 10.1016/j.tig.2011.05.005 }}</ref><ref>{{cite journal |author1=Renner, S. S. |author2=Heinrichs, J. |author3=Sousa, A. |year=2017 |title= The sex chromosomes of bryophytes: Recent insights, open questions, and reinvestigations of Frullania dilatata and Plagiochila asplenioides. |journal=Journal of Systematics and Evolution |volume=55 |issue=4 |pages=333–339 |doi=10.1111/jse.12266|doi-access=free }}</ref>

=== Mating types ===
{{Main|Mating type}}
The [[mating type]] in [[microorganism]]s is analogous to sex in multi-cellular organisms, and is sometimes described using those terms, though they are not necessarily correlated with physical body structures. Some species have more than two mating types. ''[[Tetrahymena]],'' a type of [[ciliate]], has 7 mating types

Mating types are extensively studied in fungi. Among fungi, mating type is determined by chromosomal regions called [[Mating-type locus|mating-type loci]]. Furthermore, it is not as simple as "two different mating types can mate", but rather, a matter of combinatorics. As a simple example, most ''[[Basidiomycota|basidiomycete]]'' have a "tetrapolar [[heterothallism]]" mating system: there are two loci, and mating between two individuals is possible if the alleles on ''both'' loci are different. For example, if there are 3 alleles per locus, then there would be 9 mating types, each of which can mate with 4 other mating types.<ref>{{Cite journal |last1=Idnurm |first1=Alexander |last2=Hood |first2=Michael E. |last3=Johannesson |first3=Hanna |last4=Giraud |first4=Tatiana |date=2015-12-01 |title=Contrasted patterns in mating-type chromosomes in fungi: Hotspots versus coldspots of recombination |journal=Fungal Biology Reviews |series=Special Issue: Fungal sex and mushrooms – A credit to Lorna Casselton |volume=29 |issue=3 |pages=220–229 |doi=10.1016/j.fbr.2015.06.001 |pmid=26688691 |bibcode=2015FunBR..29..220I |issn=1749-4613|pmc=4680991 }}</ref> By multiplicative combination, it generates a vast number of mating types. For example, ''[[Schizophyllum commune]],'' a type of fungus, has <math>9 \times 32 \times 9 \times 9 = 23328</math> mating types.[[File:Haplodiploid-sex-determination-system3.png |thumb|Haplodiploid sex chromosomes]]

=== Haplodiploidy ===
{{main|Haplodiploidy}}

[[Haplodiploidy]] is found in insects belonging to [[Hymenoptera]], such as [[ant]]s and [[bee]]s. Sex determination is controlled by the [[zygosity]] of a complementary sex determiner (''csd'') locus. Unfertilized eggs develop into [[haploid]] individuals which have a single, hemizygous copy of the ''csd'' locus and are therefore males. Fertilized eggs develop into [[diploid]] individuals which, due to high variability in the ''csd'' locus, are generally heterozygous females. In rare instances diploid individuals may be homozygous, these develop into sterile males.
The gene acting as a ''csd'' locus has been identified in the [[honeybee]] and several candidate genes have been proposed as a ''csd'' locus for other Hymenopterans.<ref>{{cite journal | vauthors = Beye M, Hasselmann M, Fondrk MK, Page RE, Omholt SW | title = The gene csd is the primary signal for sexual development in the honeybee and encodes an SR-type protein | journal = Cell | volume = 114 | issue = 4 | pages = 419–429 | date = August 2003 | pmid = 12941271 | doi = 10.1016/S0092-8674(03)00606-8 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Privman E, Wurm Y, Keller L | title = Duplication and concerted evolution in a master sex determiner under balancing selection | journal = Proceedings. Biological Sciences | volume = 280 | issue = 1758 | pages = 20122968 | date = May 2013 | pmid = 23466984 | pmc = 3619454 | doi = 10.1098/rspb.2012.2968 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Miyakawa MO, Tsuchida K, Miyakawa H | title = The doublesex gene integrates multi-locus complementary sex determination signals in the Japanese ant, Vollenhovia emeryi | journal = Insect Biochemistry and Molecular Biology | volume = 94 | pages = 42–49 | date = March 2018 | pmid = 29408414 | doi = 10.1016/j.ibmb.2018.01.006 | bibcode = 2018IBMB...94...42M }}</ref>
Most females in the Hymenoptera order can decide the sex of their offspring by holding received sperm in their [[spermatheca]] and either releasing it into their oviduct or not. This allows them to create more workers, depending on the status of the colony.<ref name="van Wilgenburg-2006">{{cite journal | vauthors = van Wilgenburg E, Driessen G, Beukeboom LW | title = Single locus complementary sex determination in Hymenoptera: an "unintelligent" design? | journal = Frontiers in Zoology | volume = 3 | issue = 1 | pages = 1 | date = January 2006 | pmid = 16393347 | pmc = 1360072 | doi = 10.1186/1742-9994-3-1 | doi-access = free }}</ref>

=== Polygenic sex determination ===
Polygenic sex determination is when the sex is primarily determined by genes that occur on multiple non-[[homologous chromosome]]s. The environment may have a limited, minor influence on sex determination. Examples include African cichlid fish (''[[Maylandia|Metriaclima]] spp.''), lemmings (''[[Wood lemming|Myopus schisticolor]]''), [[swordtail fish|green swordtail]],<ref name="Schartl-2004a" /> [[Japanese rice fish|medaka]],<ref name="Schartl-2004a" /> etc. In such systems, there is typically a dominance hierarchy, where one system is dominant over another if in conflict. For example, in some species of cichlid fish from Lake Malawi, if an individual has both the XY locus (on one chromosome pair) and the WZ locus (on another chromosome pair), then the W is dominant and the individual has a female phenotype.<ref>{{Cite journal |last1=Moore |first1=Emily C. |last2=Roberts |first2=Reade B. |date=June 2013 |title=Polygenic sex determination |url=https://linkinghub.elsevier.com/retrieve/pii/S0960982213004120 |journal=Current Biology |volume=23 |issue=12 |pages=R510–R512 |doi=10.1016/j.cub.2013.04.004|pmid=23787041 |bibcode=2013CBio...23.R510M }}</ref>

The sex-determination system of [[zebrafish]] is polygenic. Juvenile zebrafishes (0–30 days after hatching) have both ovary-like tissue to testis tissue. They then develop into male or female adults, with the determination based on a complex interaction genes on multiple chromosomes, but not affected by environmental variations.<ref>{{Cite journal |last1=Liew |first1=Woei Chang |last2=Bartfai |first2=Richard |last3=Lim |first3=Zijie |last4=Sreenivasan |first4=Rajini |last5=Siegfried |first5=Kellee R. |last6=Orban |first6=Laszlo |date=2012-04-10 |editor-last=Alsina |editor-first=Berta |title=Polygenic Sex Determination System in Zebrafish |journal=PLOS ONE |volume=7 |issue=4 |pages=e34397 |doi=10.1371/journal.pone.0034397 |doi-access=free |issn=1932-6203 |pmc=3323597 |pmid=22506019|bibcode=2012PLoSO...734397L }}</ref><ref>{{Cite journal |last1=Liew |first1=Woei Chang |last2=Orbán |first2=László |date=March 2014 |title=Zebrafish sex: a complicated affair |journal=Briefings in Functional Genomics |volume=13 |issue=2 |pages=172–187 |doi=10.1093/bfgp/elt041 |issn=2041-2649 |pmc=3954038 |pmid=24148942}}</ref>

===Other chromosomal systems===
In systems with two sex chromosomes, they can be heteromorphic or homomorphic. Homomorphic sex chromosomes are almost identical in size and gene content. The two familiar kinds of sex chromosome pairs (XY and ZW) are heteromorphic. Homomorphic sex chromosomes exist among pufferfish, ratite birds, pythons, and European tree frogs. Some are quite old, meaning that there is some evolutionary force that resists their differentiation.<ref name="Wright-2016">{{Cite journal |last1=Wright |first1=Alison E. |last2=Dean |first2=Rebecca |last3=Zimmer |first3=Fabian |last4=Mank |first4=Judith E. |date=2016-07-04 |title=How to make a sex chromosome |journal=Nature Communications |volume=7 |issue=1 |pages=12087 |doi=10.1038/ncomms12087 |pmid=27373494 |pmc=4932193 |bibcode=2016NatCo...712087W |issn=2041-1723}}</ref> For example, three species of [[European tree frogs]] have homologous, homomorphic sex chromosomes, and this homomorphism was maintained for at least 5.4 million years by occasional recombination.<ref>{{Cite journal |last1=Stöck |first1=Matthias |last2=Horn |first2=Agnès |last3=Grossen |first3=Christine |last4=Lindtke |first4=Dorothea |last5=Sermier |first5=Roberto |last6=Betto-Colliard |first6=Caroline |last7=Dufresnes |first7=Christophe |last8=Bonjour |first8=Emmanuel |last9=Dumas |first9=Zoé |last10=Luquet |first10=Emilien |last11=Maddalena |first11=Tiziano |last12=Sousa |first12=Helena Clavero |last13=Martinez-Solano |first13=Iñigo |last14=Perrin |first14=Nicolas |date=2011-05-17 |editor-last=Rice |editor-first=William R. |title=Ever-Young Sex Chromosomes in European Tree Frogs |journal=PLOS Biology |volume=9 |issue=5 |pages=e1001062 |doi=10.1371/journal.pbio.1001062 |doi-access=free |issn=1545-7885 |pmc=3100596 |pmid=21629756}}</ref>

The ''[[Nematocera]]'', particularly the ''[[Black fly|Simuliids]]'' and [[Chironomidae|''Chironomus'']], have sex determination regions that are labile, meaning that one species may have the sex determination region in one chromosome, but a closely related species might have the same region moved to a different non-[[Sequence homology|homologous chromosome]]. Some species even have the sex determination region different among individuals ''within'' ''the same species'' ([[intraspecific variation]]).<ref>{{Cite journal |last1=Furman |first1=Benjamin L S |last2=Metzger |first2=David C H |last3=Darolti |first3=Iulia |last4=Wright |first4=Alison E |last5=Sandkam |first5=Benjamin A |last6=Almeida |first6=Pedro |last7=Shu |first7=Jacelyn J |last8=Mank |first8=Judith E |date=2020-06-01 |editor-last=Fraser |editor-first=Bonnie |title=Sex Chromosome Evolution: So Many Exceptions to the Rules |url=https://academic.oup.com/gbe/article/12/6/750/5823304 |journal=Genome Biology and Evolution |volume=12 |issue=6 |pages=750–763 |doi=10.1093/gbe/evaa081 |issn=1759-6653 |pmc=7268786 |pmid=32315410}}</ref><ref>{{Cite journal |last1=Martin |first1=Jon |last2=Lee |first2=B. T. O. |date=September 1984 |title=A phylogenetic study of sex determiner location in a group of Australasian Chironomus species (Diptera, Chironomidae) |url=http://link.springer.com/10.1007/BF00292396 |journal=Chromosoma |volume=90 |issue=3 |pages=190–197 |doi=10.1007/BF00292396 |issn=0009-5915}}</ref><ref>{{Cite journal |last1=Martin |first1=Jon |last2=Kuvangkadilok |first2=Chaliow |last3=Peart |first3=Dianne H. |last4=Lee |first4=Barry T. O. |date=June 1980 |title=Multiple sex determining regions in a group of related Chironomus species (Diptera:Chironomidae) |url=https://www.nature.com/articles/hdy198034 |journal=Heredity |volume=44 |issue=3 |pages=367–382 |doi=10.1038/hdy.1980.34 |issn=1365-2540}}</ref> In some species, some populations have homomorphic sex chromosomes while other populations have heteromorphic sex chromosomes.

The New Zealand frog, ''[[Hochstetter's frog|Leiopelma hochstetteri]]'', uses a [[B chromosome|supernumerary sex chromosome]]. With zero of that chromosome, the frog develops into a male. With one or more, the frog develops into a female. One female had as many as 16 of that chromosome.<ref>{{Cite journal |last=Green |first=David M. |date=1988-09-01 |title=Cytogenetics of the endemic New Zealand frog, Leiopelma hochstetteri: extraordinary supernumerary chromosome variation and a unique sex-chromosome system |url=https://doi.org/10.1007/BF00331795 |journal=Chromosoma |volume=97 |issue=1 |pages=55–70 |doi=10.1007/BF00331795 |issn=1432-0886}}</ref>

Different populations of the [[Japanese wrinkled frog|Japanese frog ''Rana rugosa'']] uses different systems. Two use homomorphic male heterogamety, one uses XX/XY, one uses ZZ/ZW. Remarkably, the X and Z chromosomes are homologous, and the Y and W as well. ''Dmrt1'' is on autosome 1 and not sex-linked. This means that an XX female individual is genetically similar to a ZZ male individual, and an XY male individual is to a ZW female individual. The mechanism behind this is yet unclear, but it is hypothesized that during its recent evolution, the XY-to-ZW transition occurred twice.<ref>{{Cite journal |last1=Uno |first1=Yoshinobu |last2=Nishida |first2=Chizuko |last3=Oshima |first3=Yuki |last4=Yokoyama |first4=Satoshi |last5=Miura |first5=Ikuo |last6=Matsuda |first6=Yoichi |last7=Nakamura |first7=Masahisa |date=June 2008 |title=Comparative chromosome mapping of sex-linked genes and identification of sex chromosomal rearrangements in the Japanese wrinkled frog (Rana rugosa, Ranidae) with ZW and XY sex chromosome systems |url=https://link.springer.com/10.1007/s10577-008-1217-7 |journal=Chromosome Research |volume=16 |issue=4 |pages=637–647 |doi=10.1007/s10577-008-1217-7 |pmid=18484182 |issn=0967-3849}}</ref><ref name="Graves-2008">{{Cite journal |last=Graves |first=Jennifer A. Marshall |date=2008-12-01 |title=Weird Animal Genomes and the Evolution of Vertebrate Sex and Sex Chromosomes |url=http://dx.doi.org/10.1146/annurev.genet.42.110807.091714 |journal=Annual Review of Genetics |volume=42 |issue=1 |pages=565–586 |doi=10.1146/annurev.genet.42.110807.091714 |issn=0066-4197}}</ref>

''[[Clarias gariepinus]]'' uses both XX/XY and ZW/ZZ system within the species, with some populations using homomorphic XX/XY while others using heteromorphic ZW/ZZ. A population in Thailand appears to use both systems simultaneously, possibly because ''C. gariepinus'' were not native to Thailand, and were introduced from different source populations which resulted in a mixture.<ref>{{Cite journal |last1=Nguyen |first1=Dung Ho My |last2=Panthum |first2=Thitipong |last3=Ponjarat |first3=Jatupong |last4=Laopichienpong |first4=Nararat |last5=Kraichak |first5=Ekaphan |last6=Singchat |first6=Worapong |last7=Ahmad |first7=Syed Farhan |last8=Muangmai |first8=Narongrit |last9=Peyachoknagul |first9=Surin |last10=Na-Nakorn |first10=Uthairat |last11=Srikulnath |first11=Kornsorn |date=2021-01-05 |title=An Investigation of ZZ/ZW and XX/XY Sex Determination Systems in North African Catfish (Clarias gariepinus, Burchell, 1822) |journal=Frontiers in Genetics |volume=11 |doi=10.3389/fgene.2020.562856 |doi-access=free |pmid=33584785 |issn=1664-8021|pmc=7874028 }}</ref>

Multiple sex chromosomes like those of platypus also occurs in bony fish.<ref>{{Cite journal |last1=Sember |first1=Alexandr |last2=Nguyen |first2=Petr |last3=Perez |first3=Manolo F. |last4=Altmanová |first4=Marie |last5=Ráb |first5=Petr |last6=Cioffi |first6=Marcelo de Bello |date=2021-09-13 |title=Multiple sex chromosomes in teleost fishes from a cytogenetic perspective: state of the art and future challenges |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=376 |issue=1833 |pages=20200098 |doi=10.1098/rstb.2020.0098 |issn=0962-8436 |pmc=8310710 |pmid=34304595}}</ref> Some moths and butterflies have <math>W_1W_2Z \text {♀}/ZZ \text{♂}</math> or <math>WZ_1Z_2 \text {♀}/Z_1Z_1Z_2Z_2 \text{♂}</math>.<ref>{{Cite journal |last1=Traut |first1=W. |last2=Sahara |first2=K. |last3=Marec |first3=F. |date=2008-01-18 |title=Sex Chromosomes and Sex Determination in Lepidoptera |url=https://doi.org/10.1159/000111765 |journal=Sexual Development |volume=1 |issue=6 |pages=332–346 |doi=10.1159/000111765 |pmid=18391545 |issn=1661-5425}}</ref>

The [[Southern platyfish#Sex|Southern platyfish]] has a complex sex determination system involving 3 sex chromosomes and 4 autosomal alleles.<ref>{{Citation |last=Kallman |first=Klaus D. |title=A New Look at Sex Determination in Poeciliid Fishes |date=1984 |work=Evolutionary Genetics of Fishes |pages=95–171 |editor-last=Turner |editor-first=Bruce J. |url=https://doi.org/10.1007/978-1-4684-4652-4_3 |access-date=2024-08-04 |place=Boston, MA |publisher=Springer US |doi=10.1007/978-1-4684-4652-4_3 |isbn=978-1-4684-4652-4}}</ref><ref name="Schartl-2004b">{{cite journal |vauthors=Schartl M |date=July 2004 |title=A comparative view on sex determination in medaka |journal=Mechanisms of Development |volume=121 |issue=7–8 |pages=639–645 |doi=10.1016/j.mod.2004.03.001 |pmid=15210173 |s2cid=17401686 |doi-access=free}}</ref>

''[[Gastrotheca pseustes]]'' has <math>XY_b \text {♀} / XY_a \text{♂}</math> [[Karyotype|C-banding]] heteromorphism, meaning that both males and females have XY chromosomes, but their Y chromosomes are different on one or more C-bands. ''[[Strabomantis biporcatus|Eleutherodactylus maussi]]'' has a <math>\mathrm{X}_1 \mathrm{X}_1 \mathrm{X}_2 \mathrm{X}_2 \text {♀} / \mathrm{X}_1 \mathrm{X}_2 \mathrm{Y} \text{♂}</math> system.<ref>{{Cite journal |last1=Schmid |first1=M. |last2=Steinlein |first2=C. |last3=Feichtinger |first3=W. |date=March 1992 |title=Chromosome banding in amphibia: XVII. First demonstration of multiple sex chromosomes in amphibians: Eleutherodactylus maussi (Anura, Leptodactylidae) |url=http://link.springer.com/10.1007/BF00346007 |journal=Chromosoma |volume=101 |issue=5–6 |pages=284–292 |doi=10.1007/BF00346007 |pmid=1576881 |issn=0009-5915}}</ref><ref>{{Cite journal |last1=Schmid |first1=M. |last2=Feichtinger |first2=W. |last3=Steinlein |first3=C. |last4=Haaf |first4=T. |last5=Schartl |first5=M. |last6=Visbal García |first6=R. |last7=Manzanilla Pupo |first7=J. |last8=Fernández Badillo |first8=A. |date=2003-08-14 |title=Chromosome banding in Amphibia: XXVI. Coexistence of homomorphic XY sex chromosomes and a derived Y-autosome translocation in Eleutherodactylus maussi (Anura, Leptodactylidae) |url=https://doi.org/10.1159/000071612 |journal=Cytogenetics and Cell Genetics |volume=99 |issue=1–4 |pages=330–343 |doi=10.1159/000071612 |pmid=12900583 |issn=0301-0171}}</ref>