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polysomy
Polysomy is a condition found in many species, including fungi, plants, insects, and mammals, in which an organism has at least one more chromosome than normal, i.e., there may be three or more copies of the chromosome rather than the expected two copies. Most eukaryotic species are diploid, meaning they have two sets of chromosomes, whereas prokaryotes are haploid, containing a single chromosome in each cell. Aneuploids possess chromosome numbers that are not exact multiples of the haploid number and polysomy is a type of aneuploidy. A karyotype is the set of chromosomes in an organism and the suffix -''somy'' is used to name aneuploid karyotypes. This is not to be confused with the suffix -''ploidy'', referring to the number of complete sets of chromosomes.
Polysomy is usually caused by non-disjunction (the failure of a pair of homologous chromosomes to separate) during meiosis, but may also be due to a translocation mutation(a chromosome abnormality caused by rearrangement of parts between nonhomologous chromosomes). Polysomy is found in many diseases, including Down syndrome in humans where affected individuals possess three copies (trisomy) of chromosome 21.〔
Polysomic inheritance occurs during meiosis when chiasmata form between more than two homologous partners, producing multivalent chromosomes.〔 Autopolyploids may show polysomic inheritance of all the linkage groups, and their fertility may be reduced due unbalanced chromosome numbers in the gametes.〔 In tetrasomic inheritance, four copies of a linkage group rather than two (tetrasomy) assort two-by-two.〔
==Types==
Polysomy types are categorized based on the number of extra chromosomes in each set, noted as a diploid (2n) with an extra chromosome of various numbers. For example, a polysomy with three chromosomes is called a trisomy, a polysomy with four chromosomes is called tetrasomy, etc:
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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