Down syndrome is a genetic condition caused by extra copies of chromosome 21. It results in certain characteristics, including some degree of cognitive disability and other developmental delays. Common physical traits include upward slanting eyes, a flattened bridge of the nose, a single crease on the palm (instead of the usual three) and decreased muscle tone. Not everyone with Down syndrome has all of these traits.

The incidence of Down syndrome in the United States is about 1 in 1,000 births. There is no association between Down syndrome and culture, ethnic group, socioeconomic status or geographic region.