Down syndrome is usually caused by an error in cell division called "nondisjunction." Nondisjunction results in an embryo with three copies of chromosome 21 instead of the usual two. Prior to or at conception, a pair of 21st chromosomes in either the sperm or the egg fails to separate. As the embryo develops, the extra chromosome is replicated in every cell of the body. This type of Down syndrome, which accounts for 95% of cases, is called Trisomy 21.
Down syndrome is usually identified at birth by the presence of certain physical traits: low muscle tone, a single deep crease across the palm of the hand, a slightly flattened facial profile and an upward slant to the eyes.
The effects of Down syndrome usually change over time. Although the specific effects vary with each child, it may help you to be familiar with some general patterns of Down syndrome.
Although having Down syndrome presents some challenges, most people with the condition can flourish and live healthy, happy, and productive lives.
October 2009
Sources: Sources: Healthwise and National Down Syndrome Society
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