Down syndrome is common.
Down syndrome . 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 . Down syndrome is one of the most common chromosome abnormalities in humans . In 2015 , Down syndrome was present in 5.4 million individuals and resulted in 27,000 deaths down from 43,000 deaths in 1990 .