The causes of Down syndrome, a genetic disorder occurring in approximately one in every 800 births, have been traced to aberrant cell division. There are three different kinds of irregular cell division that are known to be among the causes of Down syndrome. Each of the three types of abnormalities is linked to additional genetic material being present on chromosome number 21. As far as scientists know, chromosome 21 is solely responsible for the development of Down syndrome.
Trisomy 21, one of the three genetic abnormalities known to be causes of Down syndrome, is linked to more than 90 percent of the cases. A child born with the genetic disorder trisomy 21 has three copies of chromosome 21 instead of two in all of the cells in his or her body. This type of Down syndrome is caused by irregular cell division during the development of either the egg or the sperm prior to fertilization; somehow either the egg or the sperm doesn’t divide correctly. In almost all cases of Down syndrome caused by the trisomy 21 disorder, the extra chromosome comes from the egg of the mother and not the father’s sperm.
Mosaic Down syndrome is a rare form of this syndrome accounting for only 1-2 percent of the cases. In it, some of a child’s cells — but not all — have an extra copy of chromosome 21. This forms a kind of mosaic of abnormal and normal cells in the body where some cells have the normal 46 chromosomes but others have 47. Mosaic Down syndrome is caused by abnormal cell division after the egg has been fertilized.
Translocation Down syndrome is the final genetic aberration known to be one of the causes of Down syndrome. This is the sole form of Down syndrome that can be passed from parent to child. Only about 4 percent of the cases of Down syndrome are attributable to this genetic disorder, and only half of those are the result of inheritance. In translocation cases, one of the parents has some genetic material that has been rearranged, but there is no extra chromosome in any cell. The parent does not have Down syndrome but can pass the damaged genetic material to a child who might develop the syndrome. A mother with damaged genetic material has about a 12 percent risk of passing it on, and a father has about a 3 percent chance.