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Ploidy is the number of homologous sets of chromosomes in a biological cell, each set essentially coding for all the biological traits of the organism. (Unfortunately there is no unique definition of homologous chromosomes.) 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 (sperm and oocytes) are haploid. In contrast, tetraploidy (four sets of chromosomes) is a type of polyploidy and is not uncommon among healthy plants, amphibians, reptiles, and insects of various species. The number of chromosomes in one of the mutually-homologous sets is called the monoploid number (n). 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-determining chromosomes. For example, a human cell has 46 chromosomes, which is an integer multiple of the monoploid number, 23. Aneuploidy is the state of not having euploidy. In humans, this may mean having an "extra" chromosome (such as Down syndrome), or missing a normal 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 (n) equals the haploid number (the number in a gamete, x), 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 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 chromosome from three species of organisms (n = 3x). Most 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. Plants 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 copies (homologs) of each chromosome, usually one from the mother and one from the father. The exact number may be one or two different from the 2n number and still be classified as diploidy (although with aneuploidy). Nearly all mammals are diploid organisms, although all individuals have some small fracton of cells that are polyploidy. Haplodiploidy A haplodiploid species is one in which one of the sexes 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, bees, and wasps. 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 Hymenopteran 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 is the process of creating a haploid cell from a diploid cell. This is a laboratory procedure that forces a normal cell to expel half of its chromosomal complement, leaving just one set. In mammals this renders this cell chromosomally equal to sperm or egg. This was one of the procedures used by Japanese researchers to produce Kaguya the fatherless mouse. 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, and are formed from the hybridisation of two separate species followed by their subsequent chromosome doubling. A good examples is the so-called Brassica triangle where three different parent species have hybridized in each pair combination to form three different allopolyploid species. Polyploidy occurs commonly in plants, but rarely in animals. Even in diploid organisms many somatic cells 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 such organisms tend to multiply continuously. | ||||||||
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