Down syndrome (
DS or
DNS) or
Down's syndrome, also known as
trisomy 21, is a
genetic disorder caused by the presence of all or part of a third copy of
chromosome 21. It is typically associated with
physical growth delays, characteristic
facial features, and mild to moderate
intellectual disability. The average
IQ of a young adult with Down syndrome is 50, equivalent to the mental age of an 8- or 9-year-old child, but this varies widely.