Editing Ploidy (section)

==Types of ploidy==

===Haploid and monoploid===
[[File:Difference of Haploid and Diploid Gene Regulation in Mendelian Genetics.svg|thumb|A comparison of [[sexual reproduction]] in predominantly haploid organisms and predominantly diploid organisms.<br /><br />'''1)''' A haploid organism is on the left and a diploid organism is on the right.<br />'''2 and 3)''' Haploid egg and sperm carrying the dominant purple gene and the recessive blue gene, respectively. These gametes are produced by simple mitosis of cells in the germ line.<br />'''4 and 5)''' Haploid sperm and egg carrying the recessive blue gene and the dominant purple gene, respectively. These gametes are produced by meiosis, which halves the number of chromosomes in the diploid germ cells.<br />'''6)''' The short-lived diploid state of haploid organisms, a zygote generated by the union of two haploid gametes during sex.<br />'''7)''' The diploid zygote which has just been fertilized by the union of haploid egg and sperm during sex.<br />'''8)''' Cells of the diploid structure quickly undergo meiosis to produce spores containing the meiotically halved number of chromosomes, restoring haploidy. These spores express either the mother's dominant gene or the father's recessive gene and proceed by mitotic division to build a new entirely haploid organism.<br />'''9)''' The diploid zygote proceeds by mitotic division to build a new entirely diploid organism. These cells possess both the purple and blue genes, but only the purple gene is expressed since it is dominant over the recessive blue gene.]]

The term '''haploid''' is used with two distinct but related definitions. In the most generic sense, haploid refers to having the number of sets of chromosomes normally found in a [[gamete]].<ref>{{cite web |title=MGI Glossary |url=http://www.informatics.jax.org/glossary/haploid |website=Mouse Genome Informatics |publisher=The Jackson Laboratory |access-date=6 July 2019 |location=Bar Harbor, Maine}}</ref> Because two gametes necessarily combine during sexual reproduction to form a single zygote from which somatic cells are generated, healthy gametes always possess exactly half the number of sets of chromosomes found in the somatic cells, and therefore "haploid" in this sense refers to having exactly half the number of sets of chromosomes found in a somatic cell. By this definition, an organism whose gametic cells contain a single copy of each chromosome (one set of chromosomes) may be considered haploid while the somatic cells, containing two copies of each chromosome (two sets of chromosomes), are diploid. This scheme of diploid somatic cells and haploid gametes is widely used in the animal kingdom and is the simplest to illustrate in diagrams of genetics concepts. But this definition also allows for haploid gametes with ''more than one'' set of chromosomes. As given above, gametes are by definition haploid, regardless of the actual number of sets of chromosomes they contain. An organism whose somatic cells are tetraploid (four sets of chromosomes), for example, will produce gametes by meiosis that contain two sets of chromosomes. These gametes might still be called haploid even though they are numerically diploid.{{citation needed|date=May 2023}}

An alternative usage defines "haploid" as having a single copy of each chromosome – that is, one and only one set of chromosomes.<ref>{{cite web |title=Talking Glossary of Genetic Terms |url=https://www.genome.gov/genetics-glossary/haploid |website=National Human Genome Research Institute |access-date=6 July 2019}}</ref> In this case, the nucleus of a [[eukaryote|eukaryotic]] cell is said to be haploid only if it has a single set of [[chromosome]]s, each one not being part of a pair. By extension a cell may be called haploid if its nucleus has one set of chromosomes, and an organism may be called haploid if its body cells (somatic cells) have one set of chromosomes per cell. By this definition haploid therefore would not be used to refer to the gametes produced by the tetraploid organism in the example above, since these gametes are numerically diploid. The term '''monoploid''' is often used as a less ambiguous way to describe a single set of chromosomes; by this second definition, haploid and monoploid are identical and can be used interchangeably.{{citation needed|date=May 2023}}

[[Gamete]]s ([[sperm]] and [[egg cell|ova]]) are haploid cells. The haploid gametes produced by most organisms combine to form a [[zygote]] with ''n'' pairs of chromosomes, i.e. 2''n'' chromosomes in total. The chromosomes in each pair, one of which comes from the sperm and one from the egg, are said to be [[homologous chromosomes|homologous]]. Cells and organisms with pairs of homologous chromosomes are called diploid. For example, most animals are diploid and produce haploid gametes. During [[meiosis]], sex cell precursors have their number of chromosomes halved by randomly "choosing" one member of each pair of chromosomes, resulting in haploid gametes. Because homologous chromosomes usually differ genetically, gametes usually differ genetically from one another.<ref>{{Cite web |date=23 September 2021 |title=Homologous chromosomes |url=https://www.genomicseducation.hee.nhs.uk/glossary/homologous-chromosomes/ |access-date=10 March 2023 |website=Genomics Education Programme}}</ref>

All [[plant]]s and many [[fungus|fungi]] and [[alga]]e switch between a haploid and a diploid state, with one of the stages emphasized over the other. This is called [[alternation of generations]]. Most fungi and algae are haploid during the principal stage of their life cycle, as are some primitive plants like [[moss]]es. More recently evolved plants, like the [[gymnosperm]]s and [[angiosperm]]s, spend the majority of their life cycle in the diploid stage. Most animals are diploid, but male [[bees]], [[wasps]], and [[ants]] are haploid organisms because they develop from unfertilized, haploid eggs, while females (workers and queens) are diploid, making their system [[haplodiploid]].{{cn|date=December 2023}}

In some cases there is evidence that the ''n'' chromosomes in a haploid set have resulted from duplications of an originally smaller set of chromosomes. This "base" number – the number of apparently originally unique chromosomes in a haploid set – is called the '''monoploid number''',<ref>Langlet, 1927.</ref> also known as '''basic''' or '''cardinal number''',<ref>Winge, 1917.</ref> or '''fundamental number'''.<ref>Manton, 1932.</ref><ref>{{cite journal | author = Fabbri F | year = 1963 | title = Primo supplemento alle tavole cromosomiche delle Pteridophyta di Alberto Chiarugi | url = https://books.google.com/books?id=2ZTzAAAAMAAJ&q=%22fundamental+number%22 | journal = Caryologia | volume = 16 | pages = 237–335 }}</ref> As an example, the chromosomes of [[common wheat]] are believed to be derived from three different ancestral species, each of which had 7 chromosomes in its haploid gametes. The monoploid number is thus 7 and the haploid number is 3&nbsp;×&nbsp;7&nbsp;= 21. In general ''n'' is a multiple of ''x''. The somatic cells in a wheat plant have six sets of 7 chromosomes: three sets from the egg and three sets from the sperm which fused to form the plant, giving a total of 42 chromosomes. As a formula, for wheat 2''n''&nbsp;= 6''x''&nbsp;= 42, so that the haploid number ''n'' is 21 and the monoploid number ''x'' is 7. The gametes of common wheat are considered to be haploid, since they contain half the genetic information of somatic cells, but they are not monoploid, as they still contain three complete sets of chromosomes (''n''&nbsp;=&nbsp;3''x'').<ref>{{cite web|url=http://mcb.berkeley.edu/courses/mcb142/lecture%20topics/Amacher/LECTURE_10_CHROM_F08.pdf|title=LECTURE 10: CHANGES IN CHROMOSOME NUMBER|website=Mcb.berkeley.edu|access-date=2022-03-10}}</ref>

In the case of wheat, the origin of its haploid number of 21 chromosomes from three sets of 7 chromosomes can be demonstrated. In many other organisms, although the number of chromosomes may have originated in this way, this is no longer clear, and the monoploid number is regarded as the same as the haploid number. Thus in humans, ''x''&nbsp;=&nbsp;''n''&nbsp;=&nbsp;23.

=== Diploid ===
{{redirect|Diploid|the geometrical construct|Dyakis dodecahedron}}
[[File:Human karyotype with bands and sub-bands.png|thumb|[[karyotype|Karyogram]] of a typical human cell, showing a diploid set of 22 [[Homologous chromosome|homologous]] [[autosomal]] chromosome pairs. It also shows both the female (XX) and male (XY) versions of the two [[sex chromosome]]s (at bottom right), as well as the [[human mitochondrial genetics|mitochondrial genome]] (to scale at bottom left). {{further|Karyotype}}]]
Diploid describes a cell or nucleus which contains two copies of genetic material, or a complete set of chromosomes, paired with their homologs (chromosome carrying the same information from the other parent).<ref>{{cite web |title=Diploid Definition |url=https://biologydictionary.net/diploid/ |website=Biology Dictionary |access-date=28 January 2025}}</ref>
'''Diploid''' cells have two [[Homologous chromosome|homologous]] copies of each [[chromosome]], usually one from the [[mother]] and one from the [[father]]. All or nearly all mammals are diploid organisms. The suspected tetraploid (possessing four-chromosome sets) plains viscacha rat (''[[Tympanoctomys barrerae]]'') and golden viscacha rat (''[[Pipanacoctomys aureus]]'')<ref name="Gallardo-2006">{{cite journal |vauthors=Gallardo MH, González CA, Cebrián I | title=Molecular cytogenetics and allotetraploidy in the red vizcacha rat, ''Tympanoctomys barrerae'' (Rodentia, Octodontidae)] | journal=Genomics | volume=88 | issue=2 | pages=214–221 | year=2006 | pmid=16580173 | doi=10.1016/j.ygeno.2006.02.010 | doi-access=free }}</ref> have been regarded as the only known exceptions (as of 2004).<ref>{{cite journal | author=Gallardo M. H. | year=2004 | title=Whole-genome duplications in South American desert rodents (Octodontidae) | journal=Biological Journal of the Linnean Society | volume=82 | issue= 4| pages=443–451 | doi=10.1111/j.1095-8312.2004.00331.x |display-authors=etal| doi-access=free | hdl=11336/102012 | hdl-access=free }}</ref> However, some genetic studies have rejected any [[polyploid]]ism in mammals as unlikely, and suggest that amplification and dispersion of repetitive sequences best explain the large genome size of these two rodents.<ref name="Svartman-2005">{{cite journal |doi=10.1016/j.ygeno.2004.12.004 |title=Molecular cytogenetics discards polyploidy in mammals |year=2005 |last1=Svartman |first1=Marta |last2=Stone |first2=Gary |last3=Stanyon |first3=Roscoe |journal=Genomics |volume=85 |issue=4 |pages=425–430 |pmid=15780745}}</ref> All normal diploid individuals have some small fraction of cells that display [[polyploid]]y. [[Human genome|Human]] diploid cells have 46 chromosomes (the [[Somatic (biology)|somatic]] number, ''2n'') and human haploid [[gametes]] (egg and sperm) have 23 chromosomes (''n''). [[Retrovirus]]es that contain two copies of their RNA genome in each viral particle are also said to be diploid. Examples include [[human foamy virus]], [[human T-lymphotropic virus]], and [[HIV]].<ref>{{Cite web |url=http://web.uct.ac.za/depts/mmi/jmoodie/hiv2.html |title=Human Retroviruses |access-date=2008-05-14 |archive-url=https://archive.today/20030330094545/http://web.uct.ac.za/depts/mmi/jmoodie/hiv2.html |archive-date=2003-03-30 |url-status=dead }}</ref>

=== Polyploidy ===
{{main |Polyploidy}}
'''Polyploidy''' is the state where all cells have multiple sets of chromosomes beyond the basic set, usually 3 or more. Specific terms are '''triploid''' (3 sets), '''tetraploid''' (4 sets), pentaploid (5 sets), hexaploid (6 sets), heptaploid<ref name="Murty-1973"/> or septaploid<ref name="Tateoka-1975"/> (7 sets), octoploid (8 sets), nonaploid (9 sets), decaploid (10 sets), undecaploid (11 sets), dodecaploid (12 sets), tridecaploid (13 sets), tetradecaploid (14 sets), etc.<ref name="Dierschke-2009">{{cite journal |vauthors=Dierschke T, Mandáková T, Lysak MA, Mummenhoff K | title=A bicontinental origin of polyploid Australian/New Zealand ''Lepidium'' species (Brassicaceae)? Evidence from genomic in situ hybridization | journal=Annals of Botany | volume=104 | issue=4 | pages=681–688 | date=September 2009 | pmid=19589857 | pmc=2729636 | doi=10.1093/aob/mcp161 }}</ref><ref name="Renny-Byfield-2010">{{cite journal |url= |title=Flow cytometry and GISH reveal mixed ploidy populations and Spartina nonaploids with genomes of ''S. alterniflora'' and ''S. maritima'' origin |author=Simon Renny-Byfield |journal=Annals of Botany |year=2010 |volume=105 |issue=4 |pages=527–533 |doi=10.1093/aob/mcq008 |display-authors=etal |pmid=20150197 |pmc=2850792}}</ref><ref name="Hummer-2009">{{cite journal |title=Decaploidy in ''Fragaria iturupensis'' (Rosaceae) |author=Kim E. Hummer |journal=Am. J. Bot. |date=March 2009 |volume=96 |number=3 |pages=713–716 |doi=10.3732/ajb.0800285 |pmid=21628226 |display-authors=etal|doi-access=free }}</ref><ref name="Talyshinskiĭ-1990">{{cite journal |url=http://www.cabdirect.org/abstracts/19911699005.html |title=Study of the fractional composition of the proteins in the compound fruit of polyploid mulberry |author=Talyshinskiĭ, G. M. |journal=Shelk |year=1990 |number=5 |pages=8–10}}</ref> Some higher ploidies include hexadecaploid (16 sets), dotriacontaploid (32 sets), and tetrahexacontaploid (64 sets),<ref>{{cite journal |url=https://www.jstage.jst.go.jp/article/csf/26/5/26_5_263/_pdf |title=Temperature dependence in Proliferation of tetraploid Meth-A cells in comparison with the parent diploid cells |author=Fujikawa-Yamamoto K |journal=Cell Structure and Function |volume=26 |issue=5 |pages=263–269 |year=2001 |doi=10.1247/csf.26.263|pmid=11831358 |doi-access=free }}</ref> though Greek terminology may be set aside for readability in cases of higher ploidy (such as "16-ploid").<ref name="Hummer-2009" /> [[Polytene chromosomes]] of plants and fruit flies can be 1024-ploid.<ref>{{cite book |url=https://books.google.com/books?id=Nrjn1kzN0_0C&pg=PA68 |title=Plant Chromosomes: Laboratory Methods |author1=Kiichi Fukui |author2=Shigeki Nakayama |isbn=9780849389191 |date=1996 |publisher=CRC Press }}</ref><ref>{{cite web |url=http://cricket.bio.indiana.edu/allied-data/lk/interactive-fly/aimorph/puffing.htm |title=Genes involved in tissue and organ development: Polytene chromosomes, endoreduplication and puffing |publisher=The Interactive Fly |access-date=2012-12-16 |archive-url=https://web.archive.org/web/20050504235437/http://cricket.bio.indiana.edu/allied-data/lk/interactive-fly/aimorph/puffing.htm |archive-date=2005-05-04 |url-status=dead }}</ref> Ploidy of systems such as the [[salivary gland]], [[elaiosome]], [[endosperm]], and [[trophoblast]] can exceed this, up to 1048576-ploid in the silk glands of the commercial silkworm ''[[Bombyx mori]]''.<ref name="D'Amato-2002" />

The chromosome sets may be from the same species or from closely related species. In the latter case, these are known as allopolyploids (or amphidiploids, which are allopolyploids that behave as if they were normal diploids). Allopolyploids are formed from the hybridization of two separate species. In plants, this probably most often occurs from the pairing of meiotically unreduced [[gametes]], and not by diploid–diploid hybridization followed by chromosome doubling.<ref>{{cite journal |last1=Ramsey |first1=Justin |last2=Schemske |first2=Douglas W. |title=Neopolyploidy in Flowering Plants |journal=Annual Review of Ecology and Systematics |date=November 2002 |volume=33 |issue=1 |pages=589–639 |doi=10.1146/annurev.ecolsys.33.010802.150437 |bibcode=2002AnRES..33..589R }}</ref> The so-called [[Triangle of U|''Brassica'' triangle]] is an example of allopolyploidy, where three different parent species have hybridized in all possible pair combinations to produce three new species.<ref>{{Cite journal |last1=Song |first1=Xiaoming |last2=Wei |first2=Yanping |last3=Xiao |first3=Dong |last4=Gong |first4=Ke |last5=Sun |first5=Pengchuan |last6=Ren |first6=Yiming |last7=Yuan |first7=Jiaqing |last8=Wu |first8=Tong |last9=Yang |first9=Qihang |last10=Li |first10=Xinyu |last11=Nie |first11=Fulei |last12=Li |first12=Nan |last13=Feng |first13=Shuyan |last14=Pei |first14=Qiaoying |last15=Yu |first15=Tong |date=2021-02-04 |title=Brassica carinata genome characterization clarifies U's triangle model of evolution and polyploidy in Brassica |journal=Plant Physiology |volume=186 |issue=1 |pages=388–406 |doi=10.1093/plphys/kiab048 |issn=0032-0889 |pmc=8154070 |pmid=33599732}}</ref>

Polyploidy occurs commonly in plants, but rarely in animals. Even in diploid organisms, many [[somatic cell]]s are polyploid due to a process called [[endoreduplication]], where duplication of the [[genome]] occurs without [[mitosis]] (cell division). The extreme in polyploidy occurs in the [[fern]] genus ''[[Ophioglossum]]'', the adder's-tongues, in which polyploidy results in chromosome counts in the hundreds, or, in at least one case, well over one thousand.{{citation needed|date=May 2023}}

It is possible for polyploid organisms to revert to lower ploidy by [[haploidisation]].{{citation needed|date=May 2023}}

====In bacteria and archaea====
[[Polyploid]]y is a characteristic of the bacterium ''[[Deinococcus radiodurans]]'' <ref name="Hansen-1978">{{cite journal |vauthors=Hansen MT |title=Multiplicity of genome equivalents in the radiation-resistant bacterium Micrococcus radiodurans |journal=J. Bacteriol. |volume=134 |issue=1 |pages=71–75 |year=1978 |pmid=649572 |pmc=222219 |doi= 10.1128/JB.134.1.71-75.1978}}</ref> and of the [[archaea|archaeon]] ''[[Halobacterium salinarum]]''.<ref name="Soppa-2011">{{cite journal |vauthors=Soppa J |title=Ploidy and gene conversion in Archaea |journal=Biochem. Soc. Trans. |volume=39 |issue=1 |pages=150–154 |year=2011 |pmid=21265763 |doi=10.1042/BST0390150 |s2cid=31385928 }}</ref> These two species are highly resistant to [[ionizing radiation]] and [[desiccation]], conditions that induce [[DNA]] double-strand breaks.<ref name="Zahradka-2006">{{cite journal |vauthors=Zahradka K, Slade D, Bailone A, Sommer S, Averbeck D, Petranovic M, Lindner AB, Radman M |title=Reassembly of shattered chromosomes in Deinococcus radiodurans |journal=Nature |volume=443 |issue=7111 |pages=569–573 |year=2006 |pmid=17006450 |doi=10.1038/nature05160 |bibcode=2006Natur.443..569Z |s2cid=4412830 }}</ref><ref name="Kottemann-2005">{{cite journal |vauthors=Kottemann M, Kish A, Iloanusi C, Bjork S, DiRuggiero J |title=Physiological responses of the halophilic archaeon Halobacterium sp. strain NRC1 to desiccation and gamma irradiation |journal=Extremophiles |volume=9 |issue=3 |pages=219–227 |year=2005 |pmid=15844015 |doi=10.1007/s00792-005-0437-4 |s2cid=8391234 |url=https://hal-mnhn.archives-ouvertes.fr/mnhn-02862359/file/2005_Kottemann_Kish_NRC1%20physiological%20response%20to%20desiccation%20and%20gamma%20radiation.pdf }}</ref> This resistance appears to be due to efficient [[homologous recombination]]al repair.

===Variable or indefinite ploidy===
Depending on growth conditions, [[prokaryote]]s such as [[bacteria]] may have a chromosome copy number of 1 to 4, and that number is commonly fractional, counting portions of the chromosome partly replicated at a given time. This is because under exponential growth conditions the cells are able to replicate their DNA faster than they can divide.{{citation needed|date=May 2023}}

In ciliates, the macronucleus is called '''ampliploid''', because only part of the genome is amplified.<ref>Schaechter, M. ''[https://books.google.com/books?id=DJLIhDnqMk0C&pg=PA217 Eukaryotic microbes]''. Amsterdam, Academic Press, 2012, p. 217.</ref>

===Mixoploidy===
Mixoploidy is the case where two cell lines, one diploid and one polyploid, [[Chimera (genetics)|coexist within the same organism]]. Though polyploidy in humans is not viable, mixoploidy has been found in live adults and children.<ref>{{cite journal |title=Mixoploidy in humans: two surviving cases of diploid-tetraploid mixoploidy and comparison with diploid-triploid mixoploidy. |author=Edwards MJ |display-authors=etal |pmid=7810564 |doi=10.1002/ajmg.1320520314 |volume=52 |issue=3 |journal=Am J Med Genet |pages=324–330|year=1994 }}</ref> There are two types: diploid-triploid mixoploidy, in which some cells have 46 chromosomes and some have 69,<ref>{{cite journal |title=46,XX/69,XXX diploid-triploid mixoploidy with hypothyroidism and precocious puberty. |last1=Järvelä |first1=IE |last2=Salo |first2=MK |last3=Santavuori |first3=P |last4=Salonen |first4=RK |pmid=8301657 |volume=30 |issue=11 |pmc=1016611 |journal=J Med Genet |pages=966–967 |doi=10.1136/jmg.30.11.966|year=1993 }}</ref> and diploid-tetraploid mixoploidy, in which some cells have 46 and some have 92 chromosomes. It is a major topic of cytology.

===Dihaploidy and polyhaploidy===
{{distinguish |text=[[Haplodiploid sex-determination system|haplodiploidy]] (where diploid and haploid individuals are different sexes)}}
Dihaploid and polyhaploid cells are formed by [[haploidisation]] of polyploids, i.e., by halving the chromosome constitution.{{citation needed|date=May 2023}}

Dihaploids (which are diploid) are important for selective breeding of tetraploid crop plants (notably potatoes), because selection is faster with diploids than with tetraploids. Tetraploids can be reconstituted from the diploids, for example by somatic fusion.{{citation needed|date=May 2023}}

The term "dihaploid" was coined by Bender<ref>{{cite journal | author=Bender K | year=1963 | title=Über die Erzeugung und Entstehung dihaploider Pflanzen bei ''Solanum tuberosum''". | journal=Zeitschrift für Pflanzenzüchtung | volume=50 | pages=141–166 }}</ref> to combine in one word the number of genome copies (diploid) and their origin (haploid). The term is well established in this original sense,<ref>Nogler, G.A. 1984. Gametophytic apomixis. In ''Embryology of angiosperms''. Edited by B.M. Johri. Springer, Berlin, Germany. pp. 475–518.</ref><ref>* {{cite journal | author=Pehu E | year=1996 | title=The current status of knowledge on the cellular biology of potato | journal=Potato Research | volume=39 | issue= 3| pages=429–435 | doi=10.1007/bf02357948| s2cid=32122774 }}</ref> but it has also been used for doubled monoploids or [[doubled haploidy|doubled haploids]], which are homozygous and used for genetic research.<ref>* {{cite journal |author1=Sprague G.F. |author2=Russell W.A. |author3=Penny L.H. | year=1960 | title=Mutations affecting quantitative traits in the selfed progeny of double monoploid maize stocks | journal=Genetics | volume=45 | issue=7 | pages=855–866 |doi=10.1093/genetics/45.7.855 |pmid=17247970 |pmc=1210096 }}</ref>

===Euploidy and aneuploidy===
'''Euploidy''' ([[Ancient Greek|Greek]] ''eu'', "true" or "even") is the state of a cell or organism having one or more than one set of the same set of chromosomes, possibly excluding the [[sex chromosome|sex-determining chromosomes]]. For example, most human cells have 2 of each of the 23 homologous monoploid chromosomes, for a total of 46 chromosomes. A human cell with one extra set of the 23 normal chromosomes (functionally triploid) would be considered euploid. Euploid [[karyotype]]s would consequentially be a multiple of the [[Ploidy#Haploid and monoploid|haploid number]], which in humans is 23.{{citation needed|date=May 2023}}

'''[[Aneuploidy]]''' is the state where one or more individual chromosomes of a normal set are absent or present in more than their usual number of copies (excluding the absence or presence of complete sets, which is considered euploidy). Unlike euploidy, aneuploid karyotypes will not be a multiple of the haploid number. In humans, examples of aneuploidy include having a single extra chromosome (as in [[Down syndrome]], where affected individuals have three copies of chromosome 21) or missing a chromosome (as in [[Turner syndrome]], where affected individuals have only one sex chromosome). Aneuploid [[karyotype]]s are given names with the suffix ''-somy'' (rather than ''-ploidy'', used for euploid karyotypes), such as [[trisomy]] and [[monosomy]].{{cn|date=June 2024}}

===Homoploid===
Homoploid means "at the same ploidy level", i.e. having the same number of [[homologous chromosome]]s. For example, homoploid [[Hybrid (biology)|hybridization]] is hybridization where the offspring have the same ploidy level as the two parental species. This contrasts with a common situation in plants where chromosome doubling accompanies or occurs soon after hybridization. Similarly, homoploid speciation contrasts with [[polyploid speciation]].{{citation needed |date=March 2017}}

===Zygoidy and azygoidy===
Zygoidy is the state in which the chromosomes are paired and can undergo meiosis. The zygoid state of a species may be diploid or polyploid.<ref>{{cite book |publisher=Academic Press |isbn=978-0-12-017603-8 |last=Books |first=Elsevier Science & Technology |title=Advances in Genetics |date=1950}}</ref><ref name="Cosín-2011">{{cite book |doi=10.1007/978-3-642-14636-7_5  |chapter=Reproduction of Earthworms: Sexual Selection and Parthenogenesis |title=Biology of Earthworms |series=Soil Biology |year=2011 |last1=Cosín |first1=Darío J. Díaz |last2=Novo |first2=Marta |last3=Fernández |first3=Rosa |volume=24 |pages=69–86 |isbn=978-3-642-14635-0 }}</ref> In the azygoid state the chromosomes are unpaired. It may be the natural state of some asexual species or may occur after meiosis. In diploid organisms the azygoid state is monoploid. (See below for dihaploidy.)