Polysomy

Polysomy is a condition found in many species, including fungi, plants, insects, and mammals, in which an <a href="/facts/Organism/j8JkL09u">organism</a> has at least one more <a href="/facts/Chromosome/FRGf11FC">chromosome</a> than normal, i.e., there may be three or more copies of the chromosome rather than the expected two copies. Most <a href="/facts/Eukaryotic/SkwyPLMA">eukaryotic</a> species are <a href="/facts/Diploid/U8zl4AQe">diploid</a>, meaning they have two sets of chromosomes, whereas <a href="/facts/Prokaryotes/oYyvaDDH">prokaryotes</a> are <a href="/facts/Haploid/U8zl4AQe">haploid</a>, containing a single chromosome in each cell. <a href="/facts/Aneuploid/CmaGcLIY">Aneuploids</a> possess chromosome numbers that are not exact multiples of the haploid number and polysomy is a type of aneuploidy. A <a href="/facts/Karyotypes/UM8j41yA">karyotype</a> 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 <a href="/facts/Non-disjunction/s4F52EtU">non-disjunction</a> (the failure of a pair of <a href="/facts/Homologous_chromosome/hktsBhyx">homologous chromosomes</a> to separate) during <a href="/facts/Meiosis/Pj5Q6nGN">meiosis</a>, but may also be due to a <a href="/facts/Chromosomal_translocation/MKEwqpQw">translocation mutation</a> (a chromosome abnormality caused by rearrangement of parts between nonhomologous chromosomes). Polysomy is found in many diseases, including <a href="/facts/Down_syndrome/Y4XEQSMS">Down syndrome</a> in humans where affected individuals possess three copies (<a href="/facts/Trisomy/K41WeULz">trisomy</a>) of <a href="/facts/Chromosome_21_(human)/F1nQLrCp">chromosome 21</a>.
Polysomic inheritance occurs during meiosis when <a href="/facts/Chiasma_(genetics)/3lzcMHPH">chiasmata</a> 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 to unbalanced chromosome numbers in the gametes. In tetrasomic inheritance, four copies of a linkage group rather than two (<a href="/facts/Tetrasomy/54hqC18w">tetrasomy</a>) assort two-by-two.

Polysomy open-in-new

Polysomy