The set of physical abnormalities, mental and functional, resulting in trisomy 21, the presence in the genomes of three rather than two normal chromosomes 21. Down syndrome is one of the most common genetic cause of mental retardation. Physical abnormalities which together give rise to distinctive facial appearance associated with this condition include eyes with epicanthic folds upslanting (folds of skin over the inside corners), flattening the nose bridge and middle portion of the face, and tend to protrude the tongue, especially when very young. Many other minor functional abnormalities, inconsequential ears, hands and feet may also be present and stature generally is low.