Down syndrome is caused by trisomy 21, the presence of three copies of chromosome 21. The extra copy ...

Down syndrome is caused by trisomy 21, the presence of three copies of chromosome 21. The extra copy usually results from nondisjunction during meiosis.

In some cases, however, the extra copy results from a translocation of most of chromosome 21 onto chromosome 14. A person who has had such a translocation in his or her gamete-producing cells is a carrier of familial Down syndrome. The carrier is normal because he or she still has two copies of all the essential genes on chromosome 21, despite the translocation. However, the same may not be true for the carrier’s offspring.

The diagram shows the six possible gametes that a carrier of familial Down syndrome could produce.