Down Syndrome (Trisomy 21)

Down Syndrome (Trisomy 21) is the most common identifiable genetic cause of intellectual disability. It is due to the presence of all or part of a third copy of chromosome 21. Individuals with Down syndrome have a characteristic facial appearance and hypotonia in infancy. About half are born with a heart defect.


The prevalence varies in different populations, anywhere from 1 in 319 to 1 in 1000 births.[1] The incidence of Down syndrome increases significantly with maternal age.


Down syndrome is also associated with an increased risk of developing early onset Alzheimer's disease.

In the majority of cases, Down syndrome is not hereditary.