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.