Down syndrome or Down's syndrome, also known as 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 clinically described earlier in the 19th century 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 by prenatal screening. The incidence of Down syndrome is estimated at 4.6 per 10,000 births[ Down syndrome occurs in all human populations, and analogous conditions have been found in other species such as chimpanzees
