A congenital disability due to a chromosome defect in which there are three copies of chromosome no. 21 instead of the usual two (see trisomy). The affected individual has a short broad face and slanted eyes (as in the Mongolian races), short fingers, and weak muscles. Down’s syndrome can be detected before birth by amniocentesis. It is named after the British physician John Down (1828–96), who first studied the incidence of the disorder.