human chromosomal disorder that results from an extra (third) copy of chromosome (chromosome number) 13. Infants born with this disorder have profound mental retardation and severe developmental malformations that include a small head, a cleft palate and lip, tiny eyes and eye openings, extra digits on hands and feet (polydactyly), clenched fingers, central nervous system abnormalities, and defects in many internal organs. There is no cure for this disorder, and most infants die in the first few months of life. The only treatment is general supportive care. Trisomy 13 is quite rare, occurring in only one out of 20,000 live births. Its incidence increases in the offspring of women and men over the age of 32.