Down Syndrome

What Is It?

Down syndrome is a disorder caused by a problem with the chromosomes — the pieces of DNA that have the blueprint for the human body. Normally a person has two copies of each chromosome, but a person with Down syndrome has three copies of chromosome 21. The condition also is called trisomy 21.

In a few cases, the extra copy is part of another chromosome (translocation), or found in only some of the person's cells (mosaicism).

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