Down Syndrome is a type of mental retardation that is the result of an aberration in chromosome 21; an extra chromosome is present. Named for Dr. Langdon Down who first identified it in 1866 (but called it “mongoloidism”), Down syndrome is the most common form of chromosomal mental retardation. It was not until 1959, however, that the cause of Down syndrome (extra chromosomal material) was determined. There is wide variation in the amount of retardation present in individuals with Down syndrome, but, on average, they have mild to moderate cognitive and physical disabilities.

[Contrary to what most people think, the correct terminology is Down syndrome, instead of Down’s syndrome.]