trisomy /trī″səmē/ [Gk, treis + soma, body] , a chromosomal aberration characterized by the presence of one more than the normal number of chromosomes in a diploid complement. In humans the trisomic cell contains 47 chromosomes and is designated 2n + 1. The additional member can join any of the normal homologous pairs, although most human trisomies involve the small chromosomes, such as those in the E or G group or the sex chromosomes. Partial trisomy occurs when only a part of a chromosome attaches to another. In genetic nomenclature, trisomies are indicated by the exact chromosome or karyotypic group in which the addition is made, such as trisomy 13 or trisomy D. Also called trisomia. Compare monosomy. See also aneuploidy, multipolar mitosis, trisomy syndrome. −trisomic, adj.
