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What is Down syndrome?

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Quick Answer

Down syndrome is a genetic disorder that causes intellectual and developmental delays, according to the National Down Syndrome Society. It is caused by abnormal cell division that results in additional unnecessary genetic material arising from chromosome 21. Those afflicted generally have a distinct facial appearance, health concerns and intellectual or developmental deficits.

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What is Down syndrome?
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Full Answer

Down syndrome is a common genetic condition in the United States, with one out of every 691 babies afflicted by it as of 2015, according to the National Down Syndrome Society. There are around 5,000 babies born every year with the condition in the United States, and roughly 400,000 Americans have it.

Down syndrome is classified by type, and there are three types of the disorder, including trisomy 21 or nondisjunction, translocation and mosaicism. The most common type is trisomy 21, since it accounts for around 95 percent of all Down syndrome cases. It is caused by nondisjunction, or cell division error. This causes an extra chromosome to be replicated into each of the body's cells.

Translocation-type Down syndrome is found in about 4 percent of all cases, and it occurs when part of chromosome 21 breaks off and attaches to another chromosome, causing Down syndrome characteristics. Around 1 percent of all Down syndrome cases are caused by mosaicism. In this type of Down syndrome, erroneous cell division only occurs in one of the cell divisions following egg fertilization. Those with this type have fewer Down syndrome characteristics.

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