All cells in the human body comprise of 23 pairs of chromosome. Amongst these pairs of chromosomes, one chromosome from each set is derived from oneâ€™s father and the other from the mother. Down syndrome is associated with chromosome 21. Random errors in cell division result in the partial or complete presence of three chromosomes 21 instead of a normal pair. The physical and cognitive disabilities arising due to the condition are the consequence of excessive genetic material provided by the extra chromosome 21.
There are three variations of Down syndrome. The most common form of Down syndrome is referred to as Trisomy 21. Owing to errors in cell division, either of the reproductive cells, that is the sperm or the egg ends up with an extra copy of chromosome 21.
Rarely, a form of Down syndrome called “Mosaic” Down syndrome may occur when an error in cell division occurs after fertilization (1 to 2 percent of cases). These persons have some cells with an extra chromosome 21 and others with the normal number.When this abnormality occurs after fertilization, the fetus may be born with Mosaic Down syndrome.
The third variation of the disease is known as Translocation Down syndrome. In this form of the disorder, a segment of chromosome 21 gets attached to another chromosome, usually chromosome 14 either prior to or during conception. Those suffering from this disorder possess only a pair of chromosomes 21, but have some extra genetic material of the same chromosome attached with the chromosome which is the site of the translocation.
Translocation Down syndrome is the only variant of the disease that can be inherited by the offspring. Other than these three causes, the disease is not known to be caused due to any behavioral or environmental factors.
In every cell in the human body there is a nucleus, where genetic material is stored in genes. Genes carry the codes responsible for all of our inherited traits and are grouped the objects called chromosomes.