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==Overview==

'''Ploidy''' is the number of [[homologous chromosome|homologous sets of chromosomes]] in a [[biological cell]]. The ploidy of cells can vary within an organism.  In humans, most cells are diploid (containing one set of chromosomes from each parent), but sex cells ([[Spermatozoon|sperm]] and [[Ovum|egg]]) are haploid.  In contrast, tetraploidy (four sets of chromosomes) is a type of [[polyploidy]] and is common in [[plant]]s, and not uncommon in [[amphibian]]s, [[reptile]]s, and various species of [[insect]]s.

The number of chromosomes in one of the mutually-homologous sets is called the '''monoploid number''' (x).  This is the same number for every set in every cell of a given organism.

'''Euploidy''' is the state of a cell or organism having an integral multiple of the monoploid number, possibly excluding the [[sex chromosome|sex-determining chromosomes]].  For example, a human cell has 46 chromosomes, which is an [[integer]] multiple of the monoploid number, 23. A human with abnormal, but integral, multiples of this full set (e.g. 69 chromosomes) would also be considered as euploid. '''[[Aneuploidy]]''' is the state of not having euploidy.  In humans, examples include having a single extra chromosome (such as [[Down syndrome]]), or missing a chromosome (such as [[Turner syndrome]]).  Aneuploidy is not normally considered ''-ploidy'' but ''-somy'', such as trisomy or monosomy.

== Haploid and monoploidy ==

The '''haploid number''' is the number of chromosomes in a [[gamete]] of an individual.  This is distinct from the monoploid number which is the number of unique chromosomes in a single complete set.  

In humans, the monoploid number (x) equals the haploid number (the number in a gamete, n), that is, x = n = 23.  In some species (especially plants), these numbers differ. Commercial common wheat is  an [[allopolyploid]] with six sets of chromosomes, two sets coming originally from each of three different species, with six copies of chromosomes in each cell.  The gametes of common wheat are considered as haploid since they contain half the genetic information of [[somatic]] cells, but are not monoploid as they still contain three complete sets of chromosomes from the original three different species (n = 3x).

Most [[fungus|fungi]] and a few algae are normally monoploid organisms.  Male bees, wasps and ants are also monoploid. For organisms that only ever have one set of chromosomes, the term '''monoploid''' is sometimes used interchangeably with haploid, but this is no longer the preferred terminology.

[[Plant]]s and some algae switch between a haploid and a diploid or [[polyploid]] state, with one of the stages emphasized over the other. This is called [[alternation of generations]].  Most diploid organisms produce monoploid sex cells that can combine to form a diploid [[zygote]], for example animals are primarily diploid but produce monoploid gametes.  During [[meiosis]], germ cell precursors have their number of chromosomes halved by randomly "choosing" one homologue, resulting in haploid germ cells ([[sperm]] and [[ovum]]).

== Diploid ==

'''Diploid''' (2n) cells have two [[Homologous chromosome|homologous]] copies of each [[chromosome]], usually one from the [[mother]] and one from the [[father]].  The exact number of chromosomes may be one or two different from the 2n number yet the cell may still be classified as diploid (although with [[aneuploidy]]).  Nearly all mammals are diploid organisms, although all individuals have some small fraction of cells that display polyploidy.

== Haplodiploidy ==<!-- This section is linked from [[Eusociality]] -->
{{further|[[Haplo-diploid sex-determination system]]}}

A '''haplodiploid''' species is one in which one of the [[sex]]es has haploid cells and the other has diploid cells.  Most commonly, the male is haploid and the female is diploid.  In such species, the male develops from unfertilized eggs, a process called arrhenotokous [[parthenogenesis]] or simply arrhenotoky, while the female develops from fertilized eggs: the sperm provides a second set of chromosomes when it fertilizes the egg.

Haplodiploidy is found in many species of insects from the order [[Hymenoptera]], particularly ants, [[bee]]s, and [[wasp]]s. One consequence of haplodiploidy is that the relatedness of sisters to each other is higher than in diploids; this has been advanced as an explanation for the [[eusociality]] common in this order of insects as it increases the power of [[kin selection]]. This argument has been disputed on the grounds that haplodiploidy also reduces the relatedness of brothers to sisters, theoretically balancing the above effect. 
In some [[Hymenoptera]]n species, worker insects are also able to produce diploid (and therefore female) fertile offspring, which develop as normal queens. The second set of chromosomes comes not from sperm, but from one of the three polar bodies during anaphase II of [[meiosis]]. This process is called thelytokous [[parthenogenesis]] or simply thelytoky.

== Haploidisation ==

'''Haploidisation''' (haploidization) is the process of creating a haploid cell (usually from a diploid cell).

A laboratory procedure called haploidisation forces a normal cell to expel half of its chromosomal complement. In [[mammals]] this renders this cell chromosomally equal to [[Spermatozoon|sperm]] or [[Ovum|egg]]. This was one of the procedures used by Japanese researchers to produce Kaguya, a fatherless mouse.

Haploidisation sometimes occurs in plants when meiotically reduced cells (usually egg cells) develop by parthenogenesis.

== Polyploidy ==
{{main|Polyploidy}}

'''Polyploidy''' is the state where all cells have multiple pairs of chromosomes beyond the basic set. These may be from the same species or from closely related species.  In the latter case these are known as allopolyploids, amphidiploids or allotetraploids. Allopolyploids can be formed from the hybridisation of two separate species followed by their subsequent chromosome doubling.   A good example is the so-called [[Triangle of U|Brassica triangle]] where three different parent species have hybridized in each pair combination to form three different allopolyploid species. Polyploid plants are probably most often formed from the pairing of meiotically unreduced gametes (Ramsey and Schemske, 2002).

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).

== Variable or indefinite ploidy ==

Depending on growth conditions, [[prokaryotes]] 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 logarithmic growth conditions the cells are able to replicate their DNA faster than they can divide.

==Mixoploidy==

Mixoploidy refers to the presence of two cell lines, one diploid and one polyploid. Though polyploidy in humans is not viable, mixoploidy has been found in live adults and children. There are two types: diploid-triploid mixoplody, in which some cells have 46 chromosomes and some have 69, and diploid-tetraploid mixoploidy, in which some cells have 46 and some have 92 chromosomes.

== Dihaploidy and Polyhaploidy ==

Dihaploid and polyhaploid cells are formed by haploidisation of polyploids, i.e., by halving the chromosome constitution.

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.

The term “dihaploid” was coined by Bender (1963) to combine in one word the number of genome copies (diploid) and their origin (haploid). The term is well established in this original sense (e.g., Nogler 1984; Pehu 1996), but it has also been used for doubled monoploids or doubled haploids, which are homozygous and used for genetic research (Sprague et al, 1960).

== References ==

* Bender, K. 1963. “Über die Erzeugung und Enstehung dihaploider Pflanzen bei ''Solanum tuberosum''”. ''Zeitschrift für Pflanzenzüchtung'' 50: 141–166.
* Griffiths, A. J. et al. 2000. An introduction to genetic analysis, 7th ed. W. H. Freeman, New York ISBN 0-7167-3520-2
* Nogler, G.A. 1984. Gametophytic apomixis. In ''Embryology of angiosperms''. Edited by B.M. Johri. Springer, Berlin, Germany. pp. 475–518.
* Pehu, E. 1996. The current status of knowledge on the cellular biology of potato. ''Potato Research'' 39: 429–435.
* Ramsey, J., and Schemske, D.W. 2002. "Neopolyploidy in flowering plants". ''Annual Review of Ecology and Systematics'' 33: 589–639.
* Sprague, G.F., Russell, W.A., and Penny, L.H. 1960. Mutations affecting quantitative traits in the selfed progeny of double monoploid maize stocks. ''Genetics'' 45(7): 855–866.

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[[Category:Classical genetics]]
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