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Polysomy is a condition in which an 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's syndrome is an example of polysomy where affected individuals possess three copies (trisomy) of chromosome 21.Polyploidy (in Greek: πολλαπλόν - multiple) is a term applied to cells or organisms that contain more than two copies of each of their chromosomes. Where an organism is normally diploid, some spontaneous aberrations may occur which are usually caused by a hampered cell division. Polyploid types are termed corresponding to the number of chromosome sets in the nucleus: triploid (three sets; 3n), tetraploid (four sets; 4n), pentaploid (five sets; 5n), hexaploid (six sets; 6n) and so on. A haploid (n) only has one set of chromosomes. Haploidy may also occur as a normal stage in an organism's life cycle as in ferns and fungi.
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