Down's Syndrome Symptoms (the Effects of Trisomy 21)

It is now well known that the extra genes on the additional chromosome 21 are the cause of Down's syndrome. Scientists are trying to determine which of the genes on chromosome 21 cause different characteristics of the disorder when present in three copies. Some genes may be more active and others less active due to the extra copy, and some of the genes may have a stronger influence on the characteristics of Down's syndrome than others. Currently, about 400 genes on chromosome 21 have been identified, but the function of most remains unknown.

Until recently, scientists hypothesized that the most important genes involved in Down's syndrome were located in an area on chromosome 21 called the Down's syndrome critical region. However, scientists have found that genes outside this region are also important in Down's syndrome.