Down syndrome is a developmental abnormality characterized by trisomy(tripling) of human chromosome 21. The extra copy of chromosome-21 leads to an over expression of certain genes located on chromosome-21. ...

Down syndrome is a developmental abnormality characterized by trisomy(tripling) of human chromosome 21. The extra copy of chromosome-21 leads to an over expression of certain genes located on chromosome-21.

Down syndrome or trisomy 21 (usually Down's Syndrome in British English) is a genetic disorder caused by the presence of all or part of an extra 21st chromosome. It is named after John Langdon Down, the British doctor who described it in 1866.

The disorder was identified as a chromosome 21 trisomy by Jérôme Lejeune in 1959. The condition is characterized by a combination of major and minor differences in structure. Often Down syndrome is associated with some impairment of cognitive ability and physical growth as well as facial appearance. Down syndrome can be identified during pregnancy or at birth.

Individuals with Down syndrome tend to have a lower than average cognitive ability, often ranging from mild to moderate learning disabilities. A small number have severe to profound mental disability. The incidence of Down syndrome is estimated at 1 per 800 to 1,000 births, although these statistics are heavily influenced by, in particular, the age of the mother. Other factors may also play a role.

Many of the common physical features of Down syndrome also appear in people with a standard set of chromosomes. They may include a single transverse(scross) palmar crease (a single instead of a double crease across one or both palms), an almond shape to the eyes, shorter limbs, poor muscle tone, a larger than normal space between the big and second toes, and protruding tongue. Health concerns for individuals with Down syndrome include a higher risk for congenital(from birth) heart defects, gastroesophageal(digestive) reflux disease, recurrent ear infections, obstructive sleep apnea, and thyroid (a body gland secreting thyroxin hormone) dysfunctions.