Down syndrome (DS or DNS), 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 ability of an 8- or 9-year-old child, but this can vary widely
The parents of the affected individual are typically genetically normal. The extra chromosome occurs by chance. The possibility increases from less than 0.1% in 20-year-old mothers to 3% in those age 45
There is no known behavioral activity or environmental factor that changes the possibility. 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