Polysomy is a condition in which an otherwise diploid organism has at least one more chromosome than normal, i.e. the number of a particular chromosome is not diploid - there may be three or more copies of the chromosome rather than the expected two copies. 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. Down syndrome in humans is an example of polysomy where affected individuals possess three copies (trisomy) of chromosome 21.
- Rieger, R.; Michaelis, A.; Green, M.M. (1968). A glossary of genetics and cytogenetics: Classical and molecular. New York: Springer-Verlag.