Ploidy is the number of complete sets of chromosomes in a biological cell. In humans, the somatic cells that compose the body are diploid (containing two complete sets of chromosomes, one set derived from each parent), but sex cells (sperm and egg) are haploid. In contrast, tetraploidy (four sets of chromosomes) is a type of polyploidy and is common in plants, and not uncommon in amphibians, reptiles, and various species of insects.
The number of chromosomes in a single non-homologous set is called the monoploid number (x). The haploid number (n) is the number of chromosomes in a gamete of an individual. Both of these numbers apply to every cell of a given organism. For humans, x = n = 23; a diploid human cell contains 46 chromosomes: 2 complete haploid sets, or 23 homologous chromosome pairs. In some species (especially plants), x and n differ, for example common wheat is an allopolyploid with six sets of chromosomes (2n=6x), two sets coming originally from each of three different species, with six sets of chromosomes in most cells and three sets of chromosomes in the gametes.
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. 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.
As stated above, the haploid number (n) is the number of chromosomes in a gamete of an individual, and this is distinct from the monoploid number (x) which is the number of unique chromosomes in a single complete set. Gametes (germ cells, sex cells, sperm, and ova) are haploid cells. The haploid gametes produced by (most) diploid organisms are monoploid, and these can combine to form a diploid zygote. For example, most animals are diploid and produce monoploid gametes.
During meiosis, germ cell precursors have their number of chromosomes halved by randomly "choosing" one homologue, resulting in haploid gametes. Because homologous chromosomes usually differ genetically, gametes usually differ genetically from one another.
All plants and many fungi and algae switch between a haploid and a diploid state (which may be polyploid), with one of the stages emphasized over the other. This is called alternation of generations. Most fungi and algae are haploid during the principal stage of their life cycle.
In humans, the monoploid number (x) equals the haploid number (n), x = n = 23, but in some species (especially plants), these numbers differ. Common wheat has six sets of chromosomes in the somatic cells, derived from its three different ancestral species. 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 (n = 3x).
Diploid (indicated by 2x) cells have two 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 2 number yet the cell may still be classified as diploid (although with aneuploidy). Nearly all mammals are diploid organisms (the viscacha rats Pipanacoctomys aureus and Tympanoctomys barrerae are the only known exceptions as of 2004), although all individuals have some small fraction of cells that display polyploidy. Human diploid cells have 46 chromosomes and human haploid gametes (egg and sperm) have 23 chromosomes.
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 sperm or 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 is the state where all cells have multiple sets 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 (or amphidiploids, which are allopolyploids that behave as if they were normal diploids). Allopolyploids are formed from the hybridization of two separate species. In plants, this probably most often occurs from the pairing of meiotically unreduced gametes, and not by diploid–diploid hybridization followed by chromosome doubling. The so-called Brassica triangle is an example of allopolyploidy, where three different parent species have hybridized in each pair combination to produce three new 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).
The extreme in polyploidy occurs in the fern-ally genus Ophioglossum, the adder's-tongues, in which polyploidy results in chromosome counts in the hundreds, or in at least one case, well over one thousand. Interestingly, these plants seem to have simplified structures in their phenotype.
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 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 mixoploidy, 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.
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 to combine in one word the number of genome copies (diploid) and their origin (haploid). The term is well established in this original sense, but it has also been used for doubled monoploids or doubled haploids, which are homozygous and used for genetic research.