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Down's Syndrome is also called Trisomy 21 because affected people have three copies of chromosome 21. How is this possible?

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Answer:

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Explanation:

About 95 percent of the time, Down syndrome is caused by trisomy 21 — the person has three copies of chromosome 21, instead of the usual two copies, in all cells. This is caused by abnormal cell division during the development of the sperm cell or the egg cell.

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