Ploidy is the number of sets of chromosomes in the nucleus of a biological cell. Normally a gamete (sperm or egg) carries a full set of chromosomes that includes a single copy of each chromosome, as aneuploidy generally leads to severe genetic disease in the offspring. The haploid number (n) is the number of chromosomes in a gamete. Two gametes form a diploid zygote with twice this number (2n) i.e. two copies of autosomal chromosomes. However, the sex chromosomes of diploid cells (excluding pseudoautosomal regions), which are subject to sex linkage, may be considered as haploid chromosomes, since haploid is also the term used to define a set of chromosomes with only one copy in the cell. Technically, ploidy is a description of a nucleus. Though at times authors may report the total ploidy of all nuclei present within the cell membrane of a syncytium, usually the ploidy of the nuclei present will be described. For example, a fungal dikaryon with two haploid nuclei is distinguished from the diploid in which the chromosomes share a nucleus and can be shuffled together.[3] Nonetheless, because in most situations there is only one nucleus, it is commonplace to speak of the ploidy of a cell. Cells are described according to the number of sets present (the ploidy level): haploid (1 set), diploid (2 sets), triploid (3 sets), tetraploid (4 sets), pentaploid (5 sets), hexaploid (6 sets), heptaploid or septaploid (7 sets), etc. The generic term polyploid is frequently used to describe cells with three or more sets of chromosomes (triploid or higher ploidy). Because chromosome number is generally reduced only by the specialized process of meiosis, the somatic cells of the body inherit and maintain the chromosome number of the zygote. However, in many situations somatic cells double their copy number by means of endoreduplication as an aspect of cellular differentiation. For example, the hearts of two-year-old children contain 85% diploid and 15% tetraploid nuclei, but by 12 years of age the proportions become approximately equal, and adults examined contained 27% diploid, 71% tetraploid and 2% octaploid nuclei. It is also possible on rare occasions for the ploidy to increase in the germline, which can result in polyploid offspring and ultimately polyploid species. This is an important evolutionary mechanism in both plants and animals. As a result, it becomes desirable to distinguish between the ploidy of a species or variety as it presently breeds and that of an ancestor. The number of chromosomes in the ancestral (non-homologous) set is called the monoploid number (x), and is distinct from the haploid number (n) in the organism as it now reproduces. Both numbers n, and x, apply to every cell of a given organism. For humans, a diploid species, x = n = 23. A typical human somatic cell contains 46 chromosomes: 2 complete haploid sets, which make up 23 homologous chromosome pairs. But common wheat is an organism where x and n differ. It has six sets of chromosomes, two sets from each of three different diploid species that are its distant ancestors. The somatic cells are hexaploid, with six sets of chromosomes, 2n = 6x = 42. The gametes are haploid for their own species, but triploid, with three sets of chromosomes, by comparison to a probable evolutionary ancestor, einkorn wheat. The monoploid number x = 7, and the haploid number n = 21. Tetraploidy (four sets of chromosomes, 2n = 4x) is common in plants, and also occurs in amphibians, reptiles, and insects. Over evolutionary time scales in which chromosomal polymorphisms accumulate, these changes become less apparent by karyotype - for example, humans are generally regarded as diploid, but the 2R hypothesis has confirmed two rounds of whole genome duplication in early vertebrate ancestors. Ploidy can also differ with life cycle and in some insects it differs by caste. In humans, only the gametes are haploid, but in the Australian bulldog ant, Myrmecia pilosula, a haplodiploid species, haploid individuals of this species have a single chromosome, and diploid individuals have two chromosomes. Alternation of generations occurs in many plants. Some studies suggest that selection is more likely to favor diploidy in host species and haploidy in parasite species.

• published: 21 Nov 2013
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