Down syndrome is a genetic condition that can be caused by one of three chromosomal abnormalities.

• The majority of persons with Down syndrome (93-96%) have trisomy 21, a condition known as "nondisjunction", in which all body cells have three copies of the 21st chromosome. (illustrated)
• Three to four percent of the individuals with Down syndrome have translocation, a condition in which each body cell has 2 sets, plus an extra piece, of chromosome 21.
• Finally, 1-2% of all persons with Down syndrome have trisomy 21 mosaicism, a condition in which only some of the body’s cells have three copies of the 21st chromosome.

Children with Down syndrome almost always have some degree of mental retardation, but it is usually in the mild to moderate range.

There is a wide variation in the communicative and cognitive status of older children with Down syndrome. Some speak clearly and understandably, others have significant issues with grammar and articulation, and a small number depend on augmentative communication devices. Many youngsters with Down syndrome attain functional levels for reading, writing, and math; others have been less successful in these areas. While there is usually some degree of impairment in both fine and gross motor skills, many children with Down syndrome are good athletes and enthusiastic participants in community life.

Children with Down syndrome tend to have certain physical characteristics, including:

• hypotonia, or low muscle tone
• small, brachycephalic head (flat in the back at the occiput)
• flattened mid-face
• small, cup-shaped ears
• small mouth with a large tongue that may protrude
• epicanthal folds in the eyes
• excess skin folds on the back of the neck
• wide space between the first and second toes
• short limbs, hands, and fingers
• a single crease across one or both palms

The National Down Syndrome Society (NDSS) has an interesting article entitled "What Causes Down Syndrome."

For some historical perspective, you can also click here to read Dr. Leshin's article Trisomy 21: The Story of Down Syndrome