What Causes Down Syndrome?

Down syndrome is caused by extra genetic material from chromosome 21 of the 23 chromosomes. There are three different causes for down syndrome. The first is Trisomy 21, which causes over 90 percent of down syndrome cases. In this case, there are three copies of chromosome 21, and is caused by abnormal cell division during development of the egg or sperm. The second type is mosaic down syndrome. This form is very rare, and it is caused by some cells (but not all) containing extra copies of chromosome 21. It is caused by abnormal cell division after fertilization. The last type is translocation down syndrome. This is when additional material from chromosome 21 becomes attached to other chromosomes.