Ploidy

See also: List of organisms by chromosome count and Chromosome § Number in various organisms
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.[1]

Ploidy (/ˈplɔɪdi/) is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and 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 cells, tissues, and 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[2] or septaploid[3] (7 sets), etc. The generic term polyploid is often used to describe cells with three or more sets of chromosomes.[4][5]

Virtually all 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.[6] 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 insects), 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]). 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.[7] 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.[8][9]

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 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.

  1. ^ Daniel Hartl (2011). Essential Genetics: A Genomics Perspective. Jones & Bartlett Learning. p. 177. ISBN 978-0-7637-7364-9.
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  3. ^ Tuguo Tateoka (May 1975). "A contribution to the taxonomy of the Agrostis mertensii-flaccida complex (Poaceae) in Japan". Journal of Plant Research. 88 (2): 65–87. doi:10.1007/bf02491243. S2CID 38029072.
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  6. ^ D. Peter Snustad; Michael J. Simmons (2012). Principles of Genetics, 6th edition. John Wiley & Sons. p. 115. ISBN 978-0-470-90359-9.
  7. ^ Otto, Sarah P. (2007). "The Evolutionary Consequences of Polyploidy". Cell. 131 (3): 452–462. doi:10.1016/j.cell.2007.10.022. ISSN 0092-8674. PMID 17981114. S2CID 10054182.
  8. ^ Mable, B. K. (2004). "'Why polyploidy is rarer in animals than in plants': myths and mechanisms". Biological Journal of the Linnean Society. 82 (4): 453–466. doi:10.1111/j.1095-8312.2004.00332.x. ISSN 0024-4066.
  9. ^ Madlung, A (2012). "Polyploidy and its effect on evolutionary success: old questions revisited with new tools". Heredity. 110 (2): 99–104. doi:10.1038/hdy.2012.79. ISSN 0018-067X. PMC 3554449. PMID 23149459.