Condition caused by a chromosomal abnormality (the presence of an extra copy of chromosome 21), which in humans produces mental retardation; a flattened face; coarse, straight hair; and a fold of skin at the inner edge of the eye (hence the former name “mongolism”). The condition can be detected by prenatal testing.
Those afflicted are usually born to mothers over 40 (one in 100); they require special care and education. The syndrome is named for J L H Down (1828–1896), an English physician who studied it. All people with Down's syndrome who live long enough eventually develop early-onset Alzheimer's disease, a form of dementia. This fact led to the discovery in 1991 that some forms of early-onset Alzheimer's disease are caused by a gene defect on chromosome 21.