trisomy The condition of a nucleus, cell, or organism in which one of the pairs of homologous chromosomes has gained an additional chromosome, resulting in a chromosome number of 2n + 1 (see aneuploid). Trisomy is the cause of a number of human genetic abnormalities, including Down's syndrome; Patau's syndrome, in which there is an extra chromosome 13 (trisomy 13); and Edwards' syndrome, in which there is an extra chromosome 18 (trisomy 18).

trisomy The condition in which a particular chromosome is represented by 3 members (i.e. the triploid condition). Compare DISOMY, NULLISOMY, POLYSOMY, and TETRASOMY.

trisomy (try-soh-mi) n. a condition in which there is one extra chromosome present in each cell in addition to the normal (diploid) chromosome set; the cause of such disorders as Down's syndrome.
—trisomic (try-soh-mik) adj.