Mongolism is one of the most common causes of mental retardation.
The majority of Down s Syndrome patients have a moderate retardation although it can range from mild to severe. Trisomy 21 occurs in about 1 in 800 live births. This incidence increases markedly as the age of the mother increases over 35. The prevalence in children born to young mothers is 1 in 1000, while it increases to almost 1 in 40 in children born to mothers over 40. Most individuals with Down s Syndrome have characteristic features such as upward slanted eyes, broad flattened face, short neck, and a prominent tongue.