What are the genetic changes related to Down syndrome?
Down syndrome is a chromosomal condition related to chromosome 21. Most cases of Down syndrome result from trisomy 21, which means each cell in the body has three copies of chromosome 21 instead of the usual two copies. The extra genetic material disrupts the normal course of development, causing the characteristic features of Down syndrome. Although the connection between Down syndrome and Alzheimer disease is unclear, researchers believe that an extra copy of a particular gene on chromosome 21, the APP gene, may account for the increased risk. A small percentage of Down syndrome cases occur when only some of the body’s cells have an extra copy of chromosome 21. These cases are called mosaic Down syndrome. Although uncommon, 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 ca
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