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Down syndrome is a genetic abnormality resulting in the birth of person with typical skeletal appearance resembling persons native to Mongolia and China, and being mentally challenged to a variable degree, with predisposition to certain diseases, and a slightly shorter than normal life span. (We must never refer to the disorder as Mongolism). Last updated November 11, 2013

The disability is named after a physician named John Langdon Down. In the year 1866, he gave the first description of this group of diseases, which therefore bears his name. He differentiated between mental retardation due to thyroid disease (Cretinism) and the genetic disorder, which he described as "Mongoloids" because these children looked like people from Mongolia. Later, due to ethnic protests, the term "Mongoloid" was dropped and the name Down Syndrome became established.

The cause of Down Syndrome was established in 1959 by Lejeune and Patricia Jacobs, working independently. They found that it was due to triplication (instead of duplication) of the Human Chromosome number 21. (The genetic code is composed of 46 chromosomes arranged as 23 pairs). This condition is called Trisomy 21, and may be due to:

  1. Trisomy being present from inception, and present in all cells
  2. Trisomy being present from inception, but midway through development, some cells loose the third chromosome and become normal paired chromosome
  3. Normal paired chromosomes are there at inception but midway through development, some cells develop into Trisomy.
  4. A rare (3-4%) form is called translocation since the extra 21 chromosome is attached or translocated on to another chromosome, usually on chromosome 14, 21 or 22. If translocation is found in a child with Down syndrome, it is important to examine the parentsí chromosomes, since in at least one-third of the cases, a parent may be a carrier of the translocation.

The second and third conditions, where some cells have Trisomy while others have a normal duplication, is called Mosaicism.

For details on genetics, see our genetics page.

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