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.