Down's syndrome

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.

The incidence for Down's syndrome births in developed countries is one in 700 live births (2000). Mothers aged over 40 are more likely to give birth to a Down's syndrome child, and in 1995 French researchers discovered a link between Down's syndrome incidence and paternal age, with men over 40 having an increased likelihood of fathering a Down's syndrome baby.

The syndrome is named after 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.