47,XX,+21; 47,XY,+21; Trisomy 21 

Mental retardation (also called mental handicap and the UK Mental Health Act (1983) defines mental impairment and severe mental impairment) is a term for a pattern of persistently slow learning of basic motor and language skills ("milestones") during childhood, and a significantly below-normal global intellectual capacity as an adult. One common criterion for diagnosis of mental retardation is a tested intelligence quotient (IQ) below 70.

A disorder caused by the presence of an extra chromosome 21 and characterized by mental retardation and distinguishing physical features.

Mental Retardation as related to Down Syndrome