There are 3 possible genetic causes of Down syndrome:
Trisomy 21. Most often, Down syndrome is caused by an extra chromosome 21 in all cells of the affected person. In these cases, the chromosome 21 pair fails to separate during the formation of an egg (or sperm); this is called "nondisjunction." When the egg with 2 copies of chromosome 21 unites with a normal sperm with one copy of chromosome 21 to form an embryo, the resulting embryo has 3 copies of chromosome 21 instead of the normal two. The extra chromosome is then copied in every cell of the baby's body, causing the features of Down syndrome.
The cause of nondisjunction is unknown, but research has shown that it happens more often as women age. Nondisjunction is not known to be caused by anything in the environment or anything that parents do (or don't do) before or during pregnancy.
Mosaic trisomy 21. In about 1-2% of cases, only some of the cells in a person's body have an extra chromosome 21; this is called "mosaic trisomy 21". In this situation, the fertilized egg may have the right number of chromosomes, but due to a cell division error early in the development of the embryo, some cells "acquire" an extra chromosome 21. A person with mosaic trisomy 21 typically has 46 chromosomes in some cells, and 47 chromosomes (with the extra chromosome 21) in others. The features and severity in people with mosaic trisomy 21 may vary widely.
Translocation trisomy 21. About 3-4% of people with Down syndrome have cells that contain 46 chromosomes; however, there is extra chromosome 21 material attached (translocated ) onto another chromosome. For parents of a child with Down syndrome due to a translocation, there may be an increased chance of Down syndrome in future pregnancies. This is because one of the two parents may be a carrier of a balanced translocation. However, not all parents of people with translocation trisomy 21 have a translocation.
Regardless of the type of Down syndrome a person has, all people with Down syndrome have an extra, critical portion of chromosome 21 present in all or some of their cells. This extra genetic material disrupts the normal course of development, causing the characteristic features of Down syndrome.
Last updated: 4/15/2015
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