Down Syndrome Down syndrome is a combination of birth defects including some degree of mental retardation and characteristic facial features. It is also called trisomy 21. Down syndrome occurs when there is an abnormality in chromosome 21. It is found in approximately 1 out of 1000 all live births. Each year, 3,000 to 5,000 people are diagnosed with Down syndrome in the United States. The formal story began in 1866, when a physician named John Langdon Down first described a set of children with common features who were distinct from other children with mental retardation. Down was superintendent of an asylum for children with