Down syndrome, present in 1 in 650-1000 live births in the Western world, is the most common genetic cause of intellectual disabilities in humans. Down syndrome is also known as trisomy 21, after its cause: the triplication of the human chromosome 21, which results in a wide array of effects, including their characteristic facial appearance and lower IQ.
Down syndrome was named after the 19th-century physician John Langdon Haydon Down. Dr. Down tried to classify the large number of intellectually disabled that came under his observation in the Earlswood Asylum for Idiots in Surrey, UK, where he worked as medical superintendent from 1855 to 1868. In his report ‘Observations on an Ethnic Classification of Idiots’ (1866), Down especially devotes attention to, what he calls, "the great Mongolian family". Nearly one and a half century later, his description is still rather accurate, although his terminology is no longer accepted nowadays. From 1866 onwards, mongolism became increasingly known as Down syndrome.
More information about J.L.H. Down and his work? Visit the website of the The Langdon Down Museum of Learning Disability.
In 1866, Dr. Down suggested that Down syndrome was caused by tuberculosis in the parents. He was wrong, but it took almost a century to definitely refute this. In 1959, on the other side of the Channel, French researchers were studying cells of Down syndrome children. Under the microscope Lejeune, Gautier and Turpin observed a small 47th chromosome in these cells, finally proving the genetic origin of the syndrome. Later, this small chromsome turned out to be an additional copy of the human chromosome 21.
95% of the Down syndrome individuals people present a whole-chromosome trisomy: three copies of chromosome 21 as compared to two copies in the general population
5% of the Down syndrome cases is due to partial trisomies:
(1) Mosaic Down syndrome: part of the cells present three copies of chromosome 21, but not all.
(2) Translocation Down syndrome: two copies of chromosome 21 are present, as well as an extra part of chromosome 21 that is attached to another chromosome.