Down syndrome

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.

The parents of the affected individual are typically genetically normal. The extra chromosome occurs by random chance. There is no known behavior or environmental factor that changes the risk. Down syndrome can be identified during pregnancy by prenatal screening followed by diagnostic testing, or after birth by direct observation and genetic testing. Since the introduction of screening, pregnancies with the diagnosis are often terminated. Regular screening for health problems common in Down syndrome is recommended throughout the person's life.