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. Down syndrome affects an estimated 1 in 800 births or 5,400 infants in the United States each year.
In a few cases, the extra copy is part of another chromosome (translocation), or found in only some of the person's cells (mosaicism). The extra DNA makes the physical and mental characteristics of Down syndrome, which include a small head that is flattened in the back; slanted eyes; extra skin folds at the corners of the eyes; small ears, nose and mouth; big-looking tongue; short stature; small hands and feet; and some degree of mental disability.