Down syndrome is a genetic condition that causes mild to serious physical and developmental problems. It is caused by an error in cell division called "nondisjunction," which results in an embryo with three copies of chromosome 21. This extra genetic material alters the course of development and causes the characteristics associated with Down syndrome. Any one of three genetic variations can cause Down syndrome: Trisomy 21, Mosaic Down syndrome, and Translocation Down syndrome.
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Trisomy 21: About 95 percent of the time, Down syndrome is caused by trisomy 21, which means the person has three copies of chromosome 21, instead of the usual two copies, in all cells. This is caused by abnormal cell division during the development of the sperm cell or the egg cell.
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Mosaic Down syndrome: In a small percentage (less than 5%) of Down syndrome cases, most of the cells in the body have the extra chromosome, but some of them dont. This is called "mosaicism." Mosaic Down syndrome can occur when the error in cell division takes place early in development but after a normal egg and sperm unite. It can also occur early in development when some cells lose an extra chromosome 21 that was present at conception.
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Translocation Down syndrome: Sometimes, a parent who does not have Down syndrome may carry a translocation in chromosome 21 that can be passed on to children and cause Down syndrome. Studying the parents chromosomes can reveal whether this is the cause of the syndrome. A genetic counselor can assist families affected by translocation trisomy 21.
There are no known behavioral or environmental factors that cause Down syndrome. However, one factor that increases the risk for having a baby with Down syndrome is the mothers age. Women who are 35 years or older when they become pregnant are more likely to have a pregnancy affected by Down syndrome than women who become pregnant at a younger age.
