<unk>  occurs in cells and organisms when there are more than two homologous sets of chromosomes.<unk> types are labelled according to the number of chromosome sets in the nucleus:
triploid (three sets; 3x), for example the genus Tardigrada
tetraploid (four sets; 4x), for example Salmonidae fish
pentaploid (five sets; 5x)
hexaploid (six sets; 6x), for example wheat
oktoploid (eight sets; 8x), for example Acipenser (genus of sturgeon fish)
dekaploid (ten sets; 10x), for example certain strawberries
dodecaploid (twelve sets; 12x), for example the plant Celosia argentea
Most organisms are normally diploid;<unk>  may occur due to abnormal cell division. It is most commonly found in plants. Haploidy may also occur as a normal stage in an organism's life. A haploid has only one set of chromosomes.<unk>  occurs in some animals, such as goldfish, salmon, and salamanders, but is especially common among ferns and flowering plants, including both wild and cultivated species. Wheat, for example, after millennia of hybridization and modification by humans, has strains that are diploid (two sets of chromosomes), tetraploid (four sets of chromosomes) with the common name of durum or macaroni wheat, and hexaploid (six sets of chromosomes) with the common name of bread wheat. Many agriculturally important plants of the genus Brassica are also tetraploids; their relationship is described by the Triangle of U.
The occurrence of<unk>  is a mechanism of speciation and is known to have resulted in new species of the plant Salsify (also known as "goatsbeard").
Examples in animals are more common in the ‘lower’ forms such as flatworms, leeches, and brine shrimp.<unk> animals are often sterile, so they often reproduce by parthenogenesis.<unk> salamanders and lizards are also quite common and parthenogenetic. While mammalian liver cells are<unk>  rare instances of<unk> mammals are known, but most often result in prenatal death.
The only known exception to this rule is an octodontid rodent of Argentina's harsh desert regions, known as the Red Viscacha-Rat (Tympanoctomys barrerae).  This rodent is not a rat, but kin to [uinea pigs and chinchillas.  Its "new" diploid [2n] number is 102 and so its cells are roughly twice normal size. Its closest living relation is Octomys mimax, the Andean Viscacha-Rat of the same family, whose 2n=56. It is surmised that an Octomys-like ancestor produced tetraploid (i.e., 4n=112) offspring that were, by virtue of their doubled chromosomes, reproductively isolated from their parents; but that these likely survived the ordinarily catastrophic effects of<unk>  in mammals by shedding (via translocation or some similar mechanism) the "extra" set of sex chromosomes gained at this doubling.<unk>  can be induced in cell culture by some chemicals: the best known is colchicine, which can result in chromosome doubling, though its use may have other less obvious consequences as well.
There are large number of<unk> crop varieties - See<unk> Crops below.
There are few naturally occurring<unk> conifers. One example is the giant tree Sequoia sempervirens or Coast Redwood which is a hexaploid (6x) with 66 chromosomes (2n=6x=66), although the origin is unclear .
Polyploidy
Aneuploidy
Monosomy
Metaplasia
Diploblasty
Heterolysis
Hemihypertrophy
Hypoplasia