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. Down syndrome is one of the most frequent genetic causes of mental retardation. The physical abnormalities that together give rise to the distinctive facial appearance associated with this condition include upslanting eyes with epicanthic folds (folds of skin over the inner corners), flatness of the bridge of the nose and of the middle portion of the face, and a tendency to protrude the tongue, especially when very young. Many other minor, functionally inconsequential abnormalities of the ears, hands, and feet may also be present, and stature is generally reduced.