(Trisomy 21)

Down Syndrome is a genetic disorder caused by extra genetic material. It affects over 350,000 people in the United States alone and is the most common (1 in 800 live births) imbalance in the number of autosomes in people. The effects of Down Syndrome vary greatly from person to person but can include mental retardation, eyes that slant upward, and heart defects.

People with Down Syndrome have 3 copies of chromosome 21. For this reason, Down Syndrome is also called "Trisomy 21".

Where does the extra chromosome come from? In 90% of Trisomy 21 cases, the additional chromosome comes from the mother's egg.