Down syndrome is the most common and readily identifiable chromosomal condition associated with mental retardation. Down syndrome is a genetic disorder that occurs in approximately 1 in 800 live births. It is caused most often by an abnormality during cell division in gamete formation called nondysjunction. As a result, the fertilized egg will contain three copies of chromosome 21. The extra chromosome interferes with normal growth and development. Therefore, it is important for parents, health care professionals, and teachers to have a clear and accurate understanding of each child's medical concerns and level of developmental functioning. In most cases, the diagnosis of Down syndrome is made according to results from a chromosome test administered shortly after birth. Although parents of any age may have a child with Down syndrome, the incidence is higher for women over 35.

There are over 50 clinical signs of Down syndrome, but it is rare to find all or even most of them in one person.

Some common characteristics include:

• Poor muscle tone;
• Slanting eyes with folds of skin at the inner corners (called epicanthal folds);
• Hyperflexibility (excessive ability to extend the joints);
• Short, broad hands with a single crease across the palm on one or both hands;
• Broad feet with short toes;
• Flat bridge of the nose;
• Short, low-set ears;
• Short neck;
• Small head;
• Small oral cavity; and/or
• Short, high-pitched cries in infancy.

Individuals with Down syndrome are usually smaller than their non-disabled peers, and their physical as well as intellectual development is slower. Besides having a distinct physical appearance, children with Down syndrome frequently have specific health-related problems.