Down syndrome affects about 1 in every 1,000 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.
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. Amniocentesis or chorionic villus sampling can be used to detect the disorder in the fetus. Listing from Electric Library , a pay service.