Down syndrome describes a set of cognitive and physical symptoms that result from an extra copy or part of a copy of chromosome 21.
Chromosomes carry the genes that tell the body to develop in certain ways and to perform certain functions. Most cells of the human body contain 23 pairs of chromosomes—one set of chromosomes from each parent— for a total of 46 chromosomes. People with Down syndrome have either a full or partial extra copy of chromosome 21, for a total of 47 chromosomes. (The syndrome is sometimes called 47,XX,+21 or 47,XY,+21 to indicate the extra chromosome 21.)
The extra chromosome disrupts the normal course of development and results in the physical features and intellectual and developmental disabilities associated with the syndrome.1
The degree of intellectual disability in people with Down syndrome varies but is usually mild to moderate. Generally, children with Down syndrome reach key developmental milestones later than other children.2 People with the syndrome also are more likely to be born with heart abnormalities, and they are at increased risk for developing hearing and vision problems, Alzheimer disease, and other conditions.1 However, with appropriate support and treatment, many people with Down syndrome lead happy, productive lives.3 In recent decades, life expectancy for people with Down syndrome has increased dramatically, from 25 years in 1983 to 60 years today.4
Down syndrome is named after John Langdon Down, the first physician to describe the syndrome systematically.
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