Causes of Down Syndrome
Three genetic variations can cause Down syndrome. Most cases of Down syndrome (92 percent) result from trisomy 21, which means that each cell in the body has three copies of chromosome 21 instead of the usual two copies.
(Click Trisomy 21 Genetics for more information about trisomy 21.)
The cause of Down syndrome in 2 to 4 percent of cases occurs when only some of the body's cells have an extra copy of chromosome 21. These cases are called mosaic Down syndrome.
(Click Mosaic Trisomy 21 for more information on mosaic Down syndrome.)
In 3 to 4 percent of cases, Down syndrome also can occur when part of chromosome 21 becomes attached (translocated) to another chromosome before or at conception. Affected people have two copies of chromosome 21, plus extra material from chromosome 21 attached to another chromosome. These cases are called translocation Down syndrome.
(Click Translocation and Down Syndrome for more information about translocation Down syndrome.)
Regardless of the genetic variation that is the cause of Down syndrome, people with the disorder have an extra critical portion of chromosome 21 in some or all of their cells. The extra genetic material disrupts the normal course of development, causing Down syndrome symptoms.
Most cases of Down syndrome are not inherited, but occur as random events during the formation of reproductive cells (eggs and sperm). The one exception is some cases of translocation Down syndrome.
In these cases, an unaffected person can carry a rearrangement of genetic material between chromosome 21 and another chromosome. This rearrangement is called a balanced translocation because there is no extra material from chromosome 21. Although they do not have signs of Down syndrome, people who carry this type of balanced translocation are at an increased risk of having children with the condition.
(Click Down Syndrome Risk for more risk factors associated with Down syndrome.)