Down syndrome, the most common genetic condition in the United States, is also called trisomy 21. What does this mean?

There are three set of chromosome 21

Explanation:

Trisomy signifies presence of abnormal three chromosomes instead of two.

Down syndrome is also known as trisomy 21 because there are three chromomose - 21 rather than two.

There are 46 chromosome in a normal human being which constitutes 23 pairs of chromosome. However, a person suffering from down syndrome has 47 chromosome with an extra chromosome - 21.

It means that chromosome 21 has a third partial or complete copy of chromosome 21. The symptom severities vary somewhat.