Down syndrome is caused by a person having three copies of chromosome 21 instead of two copies. This is why Down syndrome is also referred to by the name Trisomy 21. The picture above shows a genetic analysis of a person with Down syndrome. You will notice that the arrow is pointing to the set of number 21 chromosomes. Instead of a pair of chromosomes, as is found in all the other chromosomes, number 21 includes three chromosomes. It is important to understand that all of the chromosomes of this person are normal. It is the fact that there is an extra chromosome that causes Down syndrome. Every cell in a person with Down syndrome will contain 47 chromosomes (with the exception of Mosaic Down syndrome which is discussed below) instead of 46 chromosomes. There are many theories about how the extra chromosome causes the effects of Down syndrome but little is currently known. Research, however, is continuing and a breakthrough may provide possible treatments to lessen the effects.
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