Down syndrome, or Down's syndrome (primarily in the United Kingdom), trisomy 21, is a chromosomal condition caused by the presence of all or part of an extra 21st chromosome. It is named after John Langdon Down, the British physician who described the syndrome in 1866.
The condition was identified as a chromosome 21 trisomy by Jérôme Lejeune in 1959. Down syndrome in a fetus can be identified with amniocentesis (with risks of fetal injury and/or miscarriage) during pregnancy, or in a baby at birth.
Down syndrome is a chromosomal condition characterized by the presence of an extra copy of genetic material on the 21st chromosome, either in whole (trisomy 21) or part (such as due to translocations).
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 733 births, although it is statistically more common with older parents (both mothers and fathers) due to increased mutagenic exposures upon some older parents' reproductive cells (however, many older parents produce children without the condition). Other factors may also play a role.
Down syndrome occurs in all human populations, and analogous effects have been found in other species such as chimpanzees and mice.
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, often ranging from mild to moderate disabilities.
Many children with Down Syndrome who have received family support, enrichment therapies, and tutoring have been known to graduate from high school and college, and enjoy employment in the work force.
The average IQ of children with Down syndrome is around 50, compared to normal children with an IQ of 100. A small number have a severe to high degree of intellectual disability.
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).
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