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any condition caused by the addition of an extra member to a normal pair of homologous autosomes or to the sex chromosomes or by the translocation of a part of one chromosome to another. Most trisomies occur as a result of complete or partial nondisjunction of the chromosomes during cell division. The more severe conditions are related to trisomies of the autosomes rather than the sex chromosomes. The most common trisomy syndromes with clearly established clinical manifestations are trisomy 8, trisomy 13, trisomy 18, trisomy 21, and trisomy 22. See also trisomy.