Down syndrome /

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 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. 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