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).