Down syndrome is a condition caused by extra genetic material that results in some degree of mental retardation and certain physical problems. Usually, people with Down syndrome have an entire extra chromosome in most of their cells, although occasionally only an extra piece of a chromosome is present.

Down syndrome is among the most common birth defects and is the most common cause of developmental delays and mental retardation in the United States. The condition was named after John Langdon Down, the 19^th century physician who first described it. Down syndrome is associated with certain facial features (e.g., flatter face, upward slanting eyes) that may seem abnormal, particularly for the child’s race.

Most people have 46 chromosomes in 23 pairs in all of their cells -- 22 regular pairs and one pair of sex chromosomes. The mother and father each contribute one chromosome of each pair. However, people with Down syndrome have triplication of chromosome 21 (an extra copy in the smallest of these chromosome pairs, chromosome 21). This extra copy may be an entire extra chromosome, or only a part of it attached to a different chromosome.

This extra genetic material found in Down syndrome can affect almost every system in the body. It almost always results in developmental delays, including mild to moderate mental retardation. The severity of mental retardation and developmental delays in children with Down syndrome varies widely. Some have a near-normal overall level of functioning, but others require substantial assistance. However, people with Down syndrome tend to demonstrate near-average levels of social functioning. In general, a child's overall functioning can be improved with proper medical management, therapy, education and a supportive home environment.

About one in every 800 newborns has Down syndrome, according to the U.S. Centers for Disease Control and Prevention (CDC). The condition occurs equally among all races and ethnic groups.