Editing Ploidy (section)

{{short description|Number of sets of chromosomes of a cell}}
{{See also|List of organisms by chromosome count|Chromosome#Number in various organisms}}
[[File:Haploid vs diploid.svg|thumb|upright|A haploid set that consists of a single complete set of chromosomes (equal to the monoploid set), as shown in the picture above, must belong to a diploid species. If a haploid set consists of two sets, it must be of a tetraploid (four sets) species.<ref name="Hartl-2011">{{cite book|author=Daniel Hartl|title=Essential Genetics: A Genomics Perspective|year=2011|publisher=Jones & Bartlett Learning|isbn=978-0-7637-7364-9|pages=177|author-link=Daniel Hartl}}</ref>]]

'''Ploidy''' ({{IPAc-en|ˈ|p|l|ɔɪ|d|i}}) is the number of complete sets of [[chromosome]]s in a [[cell (biology)|cell]], and hence the number of possible [[allele]]s for [[Autosome|autosomal]] and [[Pseudoautosomal region|pseudoautosomal]] [[genes]]. Here ''sets of chromosomes'' refers to the number of maternal and paternal chromosome copies, respectively, in each homologous chromosome pair—the form in which chromosomes naturally exist. [[Somatic cell]]s, [[Tissue (biology)|tissues]], and [[Individual#Biology|individual]] organisms can be described according to the number of sets of chromosomes present (the "ploidy level"): '''monoploid''' (1 set), '''diploid''' (2 sets), '''triploid''' (3 sets), '''tetraploid''' (4 sets), '''pentaploid''' (5 sets), '''hexaploid''' (6 sets), '''heptaploid'''<ref name="Murty-1973">{{cite journal | title=Morphology of pachytene chromosomes and its bearing on the nature of polyploidy in the cytological races of Apluda mutica L. | author=U. R. Murty | journal=Genetica | year=1973 | volume=44 | issue=2 | pages=234–243 | doi=10.1007/bf00119108 | s2cid=45850598 }}</ref> or '''septaploid'''<ref name="Tateoka-1975">{{cite journal |title=A contribution to the taxonomy of the ''Agrostis mertensii''-''flaccida'' complex (Poaceae) in Japan |author=Tuguo Tateoka |journal=Journal of Plant Research |date=May 1975 |volume=88 |issue=2 |pages=65–87 |doi=10.1007/bf02491243|bibcode=1975JPlR...88...65T |s2cid=38029072 }}</ref> (7 sets), etc. The generic term '''[[polyploidy|polyploid]]''' is often used to describe cells with three or more sets of chromosomes.<ref name="Rieger-1976">{{Cite book | author1=Rieger, R. | author2=Michaelis, A. | author3=Green, M.M. | title=Glossary of Genetics and Cytogenetics: Classical and Molecular | edition=4th | publisher=Springer-Verlag | location=Berlin/Heidelberg | year=1976 | page=434 | isbn=978-3-540-07668-1 | doi=10.1007/978-3-642-96327-8| s2cid=10163081 }}</ref><ref name="Darlington-1937">{{Cite book | author1=Darlington, C. D. (Cyril Dean) | title=Recent advances in cytology | publisher=P. Blakiston's son & co. | location=Philadelphia | year=1937 | page=60 | url=https://archive.org/details/recentadvancesin00darl}}</ref>

Virtually all [[sexual reproduction|sexually reproducing]] organisms are made up of somatic cells that are diploid or greater, but ploidy level may vary widely between different organisms, between different tissues within the same organism, and at different stages in an organism's life cycle. Half of all known plant genera contain polyploid species, and about two-thirds of all grasses are polyploid.<ref name="Snustad-2012">{{cite book|author1=D. Peter Snustad|author2=Michael J. Simmons|title=Principles of Genetics, 6th edition|publisher=John Wiley & Sons|isbn=978-0-470-90359-9|page=115|year=2012}}</ref> Many animals are uniformly diploid, though polyploidy is common in invertebrates, reptiles, and amphibians. In some species, ploidy varies between individuals of the same species (as in the [[social insect]]s), and in others entire tissues and organ systems may be polyploid despite the rest of the body being diploid (as in the mammalian [[liver]]{{Citation needed|date=May 2024}}). For many organisms, especially plants and fungi, changes in ploidy level between generations are major drivers of [[speciation]]. In mammals and birds, ploidy changes are typically fatal.<ref name="Otto-2007">{{cite journal|last1=Otto, Sarah P.|title=The Evolutionary Consequences of Polyploidy|journal=Cell|volume=131|issue=3|year=2007|pages=452–462|issn=0092-8674|doi=10.1016/j.cell.2007.10.022|pmid=17981114|s2cid=10054182|author1-link=Sarah Otto|doi-access=free}}</ref> There is, however, evidence of polyploidy in organisms now considered to be diploid, suggesting that polyploidy has contributed to evolutionary diversification in plants and animals through successive rounds of polyploidization and rediploidization.<ref name="Mable-2004">{{cite journal|last1=Mable|first1=B. K.|title='Why polyploidy is rarer in animals than in plants': myths and mechanisms|journal=Biological Journal of the Linnean Society|volume=82|issue=4|year=2004|pages=453–466|issn=0024-4066|doi=10.1111/j.1095-8312.2004.00332.x|doi-access=free}}</ref><ref name="Madlung-2012">{{cite journal|last1=Madlung|first1=A|title=Polyploidy and its effect on evolutionary success: old questions revisited with new tools|journal=Heredity|volume=110|issue=2|year=2012|pages=99–104|issn=0018-067X|doi=10.1038/hdy.2012.79|pmid=23149459|pmc=3554449}}</ref>

Humans are diploid organisms, normally carrying two complete sets of chromosomes in their somatic cells: one copy of paternal and maternal chromosomes, respectively, in each of the 23 homologous pairs of chromosomes that humans normally have. This results in two homologous pairs within each of the 23 homologous pairs, providing a full complement of 46 chromosomes. This total number of individual chromosomes (counting all complete sets) is called the '''chromosome number''' or '''chromosome complement'''. The number of chromosomes found in a single complete set of chromosomes is called the '''monoploid number''' (''x''). The '''haploid number''' (''n'') refers to the total number of chromosomes found in a [[gamete]] (a [[sperm]] or [[egg]] cell produced by [[meiosis]] in preparation for sexual reproduction). Under normal conditions, the haploid number is exactly half the total number of chromosomes present in the organism's somatic cells, with one paternal and maternal copy in each chromosome pair. For diploid organisms, the monoploid number and haploid number are equal; in humans, both are equal to 23. When a human [[germ cell]] undergoes meiosis, the diploid 46 chromosome complement is split in half to form haploid gametes. After fusion of a male and a female gamete (each containing 1 set of 23 chromosomes) during [[fertilization]], the resulting [[zygote]] again has the full complement of 46 chromosomes: 2 sets of 23 chromosomes. [[#Euploidy and aneuploidy|Euploidy and aneuploidy]] describe having a number of chromosomes that is an exact multiple of the number of chromosomes in a normal gamete; and having any other number, respectively. For example, a person with [[Turner syndrome]] may be missing one sex chromosome (X or Y), resulting in a (45,X) karyotype instead of the usual (46,XX) or (46,XY). This is a type of aneuploidy and cells from the person may be said to be aneuploid with a (diploid) chromosome complement of 45.