ploidy ploidy 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. sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively, in each homologous chromosome pair, which chromosomes naturally exist as. 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 or septaploid (7 sets), etc. the generic term polyploid is often used to describe cells with three or more chromosome sets. 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. 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. 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). 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. 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. humans are diploid organisms, normally carrying two complete sets of chromosomes in their somatic cells: two copies 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. etymology the term ploidy is a back-formation from haploidy and diploidy. "ploid" is a combination of ancient greek -πλόος (-plóos, “-fold”) and -ειδής (-eidḗs), from εἶδος (eîdos, "form, likeness"). the principal meaning of the greek word ᾰ̔πλόος (haplóos) is "single", from ἁ- (ha-, “one, same”). διπλόος (diplóos) means "duplex" or "two-fold". diploid therefore means "duplex-shaped" (compare "humanoid", "human-shaped"). polish botanist eduard strasburger coined the terms haploid and diploid in 1905. some authors suggest that strasburger based the terms on august weismann's conception of the id (or germ plasm), hence haplo-id and diplo-id. the two terms were brought into the english language from german through william henry lang's 1908 translation of a 1906 textbook by strasburger and colleagues. types of ploidy haploid and monoploid a comparison of sexual reproduction in predominantly haploid organisms and predominantly diploid organisms.1) a haploid organism is on the left and a diploid organism is on the right.2 and 3) haploid egg and sperm carrying the dominant purple gene and the recessive blue gene, respectively. these gametes are produced by simple mitosis of cells in the germ line.4 and 5) haploid sperm and egg carrying the recessive blue gene and the dominant purple gene, respectively. these gametes are produced by meiosis, which halves the number of chromosomes in the diploid germ cells.6) the short-lived diploid state of haploid organisms, a zygote generated by the union of two haploid gametes during sex.7) the diploid zygote which has just been fertilized by the union of haploid egg and sperm during sex.8) cells of the diploid structure quickly undergo meiosis to produce spores containing the meiotically halved number of chromosomes, restoring haploidy. these spores express either the mother's dominant gene or the father's recessive gene and proceed by mitotic division to build a new entirely haploid organism.9) the diploid zygote proceeds by mitotic division to build a new entirely diploid organism. these cells possess both the purple and blue genes, but only the purple gene is expressed since it is dominant over the recessive blue gene. the term haploid is used with two distinct but related definitions. in the most generic sense, haploid refers to having the number of sets of chromosomes normally found in a gamete. because two gametes necessarily combine during sexual reproduction to form a single zygote from which somatic cells are generated, healthy gametes always possess exactly half the number of sets of chromosomes found in the somatic cells, and therefore "haploid" in this sense refers to having exactly half the number of sets of chromosomes found in a somatic cell. by this definition, an organism whose gametic cells contain a single copy of each chromosome (one set of chromosomes) may be considered haploid while the somatic cells, containing two copies of each chromosome (two sets of chromosomes), are diploid. this scheme of diploid somatic cells and haploid gametes is widely used in the animal kingdom and is the simplest to illustrate in diagrams of genetics concepts. but this definition also allows for haploid gametes with more than one set of chromosomes. as given above, gametes are by definition haploid, regardless of the actual number of sets of chromosomes they contain. an organism whose somatic cells are tetraploid (four sets of chromosomes), for example, will produce gametes by meiosis that contain two sets of chromosomes. these gametes might still be called haploid even though they are numerically diploid. an alternative usage defines "haploid" as having a single copy of each chromosome – that is, one and only one set of chromosomes. in this case, the nucleus of a eukaryotic cell is said to be haploid only if it has a single set of chromosomes, each one not being part of a pair. by extension a cell may be called haploid if its nucleus has one set of chromosomes, and an organism may be called haploid if its body cells (somatic cells) have one set of chromosomes per cell. by this definition haploid therefore would not be used to refer to the gametes produced by the tetraploid organism in the example above, since these gametes are numerically diploid. the term monoploid is often used as a less ambiguous way to describe a single set of chromosomes; by this second definition, haploid and monoploid are identical and can be used interchangeably. gametes (sperm and ova) are haploid cells. the haploid gametes produced by most organisms combine to form a zygote with n pairs of chromosomes, i.e. 2n chromosomes in total. the chromosomes in each pair, one of which comes from the sperm and one from the egg, are said to be homologous. cells and organisms with pairs of homologous chromosomes are called diploid. for example, most animals are diploid and produce haploid gametes. during meiosis, sex cell precursors have their number of chromosomes halved by randomly "choosing" one member of each pair of chromosomes, 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, 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, as are some primitive plants like mosses. more recently evolved plants, like the gymnosperms and angiosperms, spend the majority of their life cycle in the diploid stage. most animals are diploid, but male bees, wasps, and ants are haploid organisms because they develop from unfertilized, haploid eggs, while females (workers and queens) are diploid, making their system haplodiploid. in some cases there is evidence that the n chromosomes in a haploid set have resulted from duplications of an originally smaller set of chromosomes. this "base" number – the number of apparently originally unique chromosomes in a haploid set – is called the monoploid number, also known as basic or cardinal number, or fundamental number. as an example, the chromosomes of common wheat are believed to be derived from three different ancestral species, each of which had 7 chromosomes in its haploid gametes. the monoploid number is thus 7 and the haploid number is 3 × 7 = 21. in general n is a multiple of x. the somatic cells in a wheat plant have six sets of 7 chromosomes: three sets from the egg and three sets from the sperm which fused to form the plant, giving a total of 42 chromosomes. as a formula, for wheat 2n = 6x = 42, so that the haploid number n is 21 and the monoploid number x is 7. the gametes of common wheat are considered to be haploid, since they contain half the genetic information of somatic cells, but they are not monoploid, as they still contain three complete sets of chromosomes (n = 3x). in the case of wheat, the origin of its haploid number of 21 chromosomes from three sets of 7 chromosomes can be demonstrated. in many other organisms, although the number of chromosomes may have originated in this way, this is no longer clear, and the monoploid number is regarded as the same as the haploid number. thus in humans, x = n = 23. diploid karyogram of a typical human cell, showing a diploid set of 22 homologous autosomal chromosome pairs. it also shows both the female (xx) and male (xy) versions of the two sex chromosomes (at bottom right), as well as the mitochondrial genome (to scale at bottom left). further information: karyotype diploid cells have two homologous copies of each chromosome, usually one from the mother and one from the father. all or nearly all mammals are diploid organisms. the suspected tetraploid (possessing four-chromosome sets) plains viscacha rat (tympanoctomys barrerae) and golden viscacha rat (pipanacoctomys aureus) have been regarded as the only known exceptions (as of 2004). however, some genetic studies have rejected any polyploidism in mammals as unlikely, and suggest that amplification and dispersion of repetitive sequences best explain the large genome size of these two rodents. all normal diploid individuals have some small fraction of cells that display polyploidy. human diploid cells have 46 chromosomes (the somatic number, 2n) and human haploid gametes (egg and sperm) have 23 chromosomes (n). retroviruses that contain two copies of their rna genome in each viral particle are also said to be diploid. examples include human foamy virus, human t-lymphotropic virus, and hiv. polyploidy main article: polyploidy polyploidy is the state where all cells have multiple sets of chromosomes beyond the basic set, usually 3