Claim: Down syndrome is a chromosome abnormality.

Down syndrome -LRB- DS or DNS -RRB- , 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 .   There is no cure for Down syndrome . Education and proper care have been shown to improve quality of life . Some children with Down syndrome are educated in typical school classes , while others require more specialized education . Some individuals with Down syndrome graduate from high school and a few attend post-secondary education . In adulthood , about 20 % in the United States do paid work in some capacity with many requiring a sheltered work environment . Support in financial and legal matters is often needed . Life expectancy is around 50 to 60 years in the developed world with proper health care .   Down syndrome is one of the most common chromosome abnormalities in humans . It occurs in about one per 1000 babies born each year . In 2015 , Down syndrome was present in 5.4 million individuals and resulted in 27,000 deaths down from 43,000 deaths in 1990 . It is named after John Langdon Down , the British doctor who fully described the syndrome in 1866 . Some aspects of the condition were described earlier by Jean-Étienne Dominique Esquirol in 1838 and Édouard Séguin in 1844 . In 1957 , the genetic cause of Down syndrome , an extra copy of chromosome 21 , was discovered .