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Ploidy is the number of complete sets of chromosomes in a biological cell. In humans, the somatic cells that comprise 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). This is the same number for every cell of a given organism. The monoploid number for humans is 23, and a diploid human cell contains 46 chromosomes: 2 complete haploid sets, or 23 homologous chromosome pairs.

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.


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