This page on the eMedTV website explores the causes of Down syndrome. Most cases are caused by trisomy 21, which means that each cell in the body has three copies of chromosome 21 instead of the usual two copies.
Trisomy 21 genetics are vital to an understanding of Down syndrome. This eMedTV article explains the link between trisomy 21 genetics and Down syndrome -- namely, the condition results when three (rather than two) copies of chromosome 21 are present.
Mosaic trisomy 21, as this eMedTV segment explains, is a genetic variation that can cause Down syndrome. In mosaic trisomy 21, the extra chromosome 21 is present in only some of the cells, which means that associated physical problems can vary.
As this eMedTV page explains, when material from one chromosome 21 is stuck onto another chromosome, translocation Down syndrome results. This is why people with translocation Down syndrome have 46 chromosomes, but still have the characteristics.
Down syndrome risk increases if one of the parents is a balanced translocation carrier. This section of the eMedTV library provides details about other Down syndrome risk factors, such as maternal age.
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