Down syndrome occurs when an individual has a full or partial extra copy of chromosome 21. This additional genetic material alters the course of development and causes the characteristics associated with Down syndrome.
There are three types of Down syndrome: trisomy 21 (nondisjunction) accounts for 95% of cases, translocation accounts for about 4% and mosaicism accounts for about 1%.
Down syndrome is the most commonly occurring chromosomal condition. One in every 691 babies in the United States is born with Down syndrome.
There are more than 400,000 people living with Down syndrome in the United States.
Down syndrome occurs in people of all races and economic levels.
There are three types of Down syndrome: trisomy 21 (nondisjunction) accounts for 95% of cases, translocation accounts for about 4% and mosaicism accounts for about 1%.