Down syndrome is the set of physical, mental, and functional abnormalities that result from trisomy 21, the presence in the genome of three rather than the normal two chromosomes 21. The physical abnormalities that together give rise to the distinctive facial appearance associated with this condition include upslanting palpebral fissures with inner epicanthic folds, flatness of the bridge of the nose, midfacial hypoplasia, and a tendency to protrude the tongue, especially when very young. Many other functionally inconsequential minor abnormalities of the ears, hands, and feet may also be present, and stature is generally reduced. Approximately 40% of affected individuals are born with congenital heart disease, with endocardial cushion and related septal defects frequently being present. Obstruction of the intestinal tract also occasionally occurs during development. Although trisomy 21 is the autosomal trisomy most compatible with survival through the period of gestation, only about a third of affected embryos and fetuses are actually liveborn.
- Pueschel SM, Rynders JE, eds (1982): Down Syndrome: Advances in Biomedicine and the Behavioral Sciences. Cambridge: Ware PressGoogle Scholar