Down syndrome is a chromosomal condition characterized by the presence of a full or partial copy of the 21st chromosome. It is named after John Langdon Down, the British physician who described the syndrome in 1866. The condition was identified as a chromosomal condition by Jérôme Lejeune in 1959.
There are three types of Down syndrome: (1) trisomy 21, which occurs when every cell has an extra copy of chromosome 21, (2) mosaicism, which occurs when only some cells contain the extra copy of chromosome 21, and (3) translocation, which occurs when cells contain the usual two pairs of chromosome 21 plus an extra part of chromosome 21. Trisomy 21 is by far the most common type of Down syndrome.
Down syndrome can be identified during pregnancy through prenatal screening tests (i.e, blood tests and sonograms) or through chorionic villus sampling (CVS) or amniocentesis, both of which carry a slight risk of fetal injury and/or miscarriage. It is identified after birth by direct observation and a chromosomal analysis of a blood sample.
The effects and extent of the extra copy vary greatly among people depending on genetic history and pure chance. The incidence of Down syndrome is estimated at 1 per 691 births, although it is statistically more common with older parents. Down syndrome occurs in all human populations.
Often Down syndrome is associated with some impairment of cognitive ability and physical growth, and a particular set of facial characteristics. Individuals with Down syndrome tend to have a lower-than-average cognitive ability that ranges from a mild to moderate disability. The average IQ of children with Down syndrome is around 50, compared to children without an intellectual disability, who have an average IQ of 100. A small number of individuals have a severe to high degree of intellectual disability. Many individuals with Down Syndrome, however, graduate from high school and college and enjoy employment in the work force.
Many of the common physical features of Down syndrome may also appear in people with a standard set of chromosomes, including microgenia (an abnormally small chin), an unusually round face, macroglossia (protruding or oversized tongue), an almond shape to the eyes caused by an epicanthic fold of the eyelid, upslanting palpebral fissures (the separation between the upper and lower eyelids), shorter limbs, a single transverse palmar crease (a single instead of a double crease across one or both palms), poor muscle tone, and a larger than normal space between the big and second toes. Health concerns for individuals with Down syndrome include a higher risk for congenital heart defects, gastroesophageal reflux disease, recurrent ear infections, obstructive sleep apnea, and thyroid dysfunctions.
Early childhood intervention, screening for common problems, medical treatment where indicated, a conducive family environment, and vocational training can improve the overall development of children with Down syndrome. Education and proper care will improve quality of life significantly, despite genetic limitations.
Advocates for people with Down syndrome point to various factors, such as additional educational support and parental support groups to improve parenting knowledge and skills. There are also strides being made in education, housing, and social settings to create environments which are accessible and supportive to people with Down syndrome. In most developed countries, since the early twentieth century many people with Down syndrome were housed in institutions or colonies and excluded from society. However, since the early 1960s parents and their organizations, educators, and other professionals have generally advocated a policy of inclusion, bringing people with any form of mental or physical disability into general society as much as possible. Such organizations include the National Association for Down Syndrome, the first known organization advocating for individuals with Down Syndrome in the United States founded by Kathryn McGee in 1960; MENCAP advocating for all with mental disabilities, which was founded in the U.K. in 1946 by Judy Fryd; and the National Down Syndrome Congress, the first truly national organization in the U.S. advocating for families of persons with Down syndrome, founded in 1973 by Kathryn McGee and others Kathryn McGee.
The first World Down Syndrome Day was held on 21 March 2006. The day and month were chosen to correspond with 21 and trisomy respectively. It was proclaimed by the European Down Syndrome Association during their European congress in Palma de Mallorca (febr. 2005). In the United States, the National Down Syndrome Society observes Down Syndrome Month every October as “a forum for dispelling stereotypes, providing accurate information, and raising awareness of the potential of individuals with Down syndrome.”