Down syndrome, congenital disorder characterized by mild to severe mental retardation, slow physical development, and characteristic physical features. Down syndrome affects about 1 in every 730 live births and occurs in all populations equally. It was first described in 1866 by an English physician, J. Langdon Down. In 1959 a French physician, Jerome Lejeune, discovered that the syndrome was caused by an extra chromosome. It was later discovered that this extra chromosome appears as a third chromosome attached to the 21st of the 23 pairs of chromosomes normally present in the human genome. This third chromosome gives rise to the alternate name trisomy 21.

The extra genetic material is responsible for the physical characteristics of the syndrome: low muscle tone, flattish facial features, an upward slant to the eyes and epicanthal folds (which were the basis for the former name, mongolism), a single crease across the palm, hyperflexibility of the joints, and a displastic middle phalanx on the fifth finger. People with Down syndrome have an increased incidence of infection, childhood leukemia, congenital heart defects, and respiratory problems, but modern medical treatment has improved the life expectancy from 9 (in 1910) to 55 (in 1995).

Mental retardation varies widely, from minimal to severe. The great majority of those who have the disorder attend public schools and as adults can live independently or in group homes. After age 35 individuals with the syndrome develop the neurological changes of Alzheimer's disease, and many develop the dementia that accompanies them.

Eighty percent of children with Down syndrome are born to women under 35 years of age, but the incidence of Down syndrome births does increase with age. Approximately 5% of cases are transmitted by the sperm. Amniocentesis or chorionic villus sampling can be used to detect the disorder in the fetus. Children born to women with Down syndrome have a 50% chance of having the disorder.

Down, district (1991 pop. 57,050), 246 sq mi (637 sq km), SE Northern Ireland. The principal town is Downpatrick. The shoreline extends along the Irish Sea and is deeply indented by Strangford Lough. The undulating surface rises to the beautiful Mourne Mts. in the south. Chief rivers are the Lagan and the Bann. Agriculture is the most important activity, and the area is extensively cultivated (oats, barley, wheat, and hay). Market gardening is common in the north. Sheep and pigs are also raised. Manufactures include linen, cotton, rayon, clothing, agricultural machinery, and metals. The chief manufacturing towns are Downpatrick and Ballynahinch. Fishing is economically important; herring and whitefish constitute the main catch. Newcastle and Killyleagh are seaside resort towns.

or trisomy 21

Congenital disorder caused by an extra chromosome (trisomy) on the chromosome 21 pair. Those with the syndrome may have broad, flat faces; up-slanted eyes, sometimes with epicanthal folds (whence its former name, mongolism); intellectual disability (usually moderate); heart or kidney malformations; and abnormal fingerprint patterns. Many persons with Down syndrome can live and work independently or in a sheltered environment, but they age prematurely and have a short (55-year) life expectancy. The risk of bearing a child with the disorder increases with the mother's age; it can be detected in the fetus by amniocentesis.

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District (pop., 1999 est.: 63,800), Northern Ireland. Formerly part of County Down, it was established as a district in 1973. It fronts Strangford Lough and the Irish Sea. Extreme southern and western Down is mountainous; the dome-shaped Mourne Mountains reach an elevation of 2,789 ft (850 m). Down is a rich agricultural district; livestock raising is important. Down was settled in prehistoric times. It was where St. Patrick began his mission in Ireland (AD 432), and his well and bathhouses are preserved near the district seat, Downpatrick. In Tudor times, parts of Down were colonized by English and Scottish adventurers.

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