... Otherwise Known as "Trisomy 21"
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In humans, the egg cells and sperm cells have 23 chromosomes. The rest of your cells normally contain 23 pairs of chromosomes — one from your father and one from your mother. Those with Down syndrome usually have three copies of chromosome 21 — called trisomy 21 — instead of two copies.


Nature of the Genetic Difficulty

Three genetic variations can cause Down syndrome. In most cases, approximately 92% of the time, Down syndrome is caused by the presence of an extra chromosome 21 in all cells of the individual. In such cases, the extra chromosome originates in the development of either the egg or the sperm. Consequently, when the egg and sperm unite to form the fertilized egg, three--rather than two--chromosomes 21 are present. As the embryo develops, the extra chromosome is repeated in every cell. This condition, in which three copies of chromosome 21 are present in all cells of the individual, is called trisomy 21. (http://www.medicinenet.com/down_syndrome/article.htm)
This extra genetic material on the 21st chromosome causes delays in the way a child develops, both mentally and physically. No one knows exactly for sure why DS occurs, and there's no way to prevent the chromosomal error that causes it. The disease can be detected even in pregnancy by a simple genetic test.

Basis Facts of Disease


Prognosis


Sources Cited