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Down syndrome (DS) or Down's syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. Trisomy 21 is the most common chromosome abnormality in humans.
The average IQ of young adults with Down syndrome is around 50, whereas young adults without the condition typically have an IQ of 100. (MR has historically been defined as an IQ below 70.) A large proportion of individuals with Down syndrome have a severe degree of intellectual disability.
Down syndrome is named after John Langdon Down, the British physician who described the syndrome in 1866. The condition was clinically described earlier by Jean Etienne Dominique Esquirol in 1838 and Edouard Seguin in 1844. Down syndrome was identified as a chromosome 21 trisomy by Dr. Jérôme Lejeune in 1959. Down syndrome can be identified in a baby at birth or before birth by prenatal screening. Pregnancies with this diagnosis are often terminated.
Many children with Down syndrome are educated in regular school classes while others require specialised educational facilities. Education and proper care has been shown to improve quality of life significantly. Many adults with Down Syndrome are able to work at paid employment in the community, while others require a more sheltered work environment.
Mental characteristics and neurology
Most individuals with Down syndrome have intellectual disability in the mild (IQ 50–70) to moderate (IQ 35–50) range, with individuals having Mosaic Down syndrome typically 10–30 points higher. The methodology of the IQ tests has been criticised for not taking into account accompanying physical disabilities, such as hearing and vision impairment, that would slow performance.
Language skills show a difference between understanding speech and expressing speech, and commonly individuals with Down syndrome have a speech delay. Fine motor skills are delayed and often lag behind gross motor skills and can interfere with cognitive development. Effects of the condition on the development of gross motor skills are quite variable.
Some children will begin walking at around 2 years of age, while others will not walk until age four. Physical therapy, and/or participation in a program of adapted physical education (APE), may promote enhanced development of gross motor skills in Down syndrome children.