Down syndrome is a common genetic disorder. Down syndrome causes birth defects, medical problems, and some degree of intellectual disability.

Chromosomes contain the genetic makeup of your body. They occur in 23 pairs within the body. Down syndrome is a problem with chromosome pair 21. There is extra genetic material on the chromosome that may be caused by:

  • An extra chromosome—This type is called Trisomy 21. This results from an error in cell division in the egg or sperm.
  • Extra chromosomes in some cells—This type is called Mosaic Trisomy 21.
  • Part of the chromosome breaking off and reattaching to another chromosome—This type is called translocation trisomy. In about one third of individuals, the translocation is inherited from a parent.