Down syndrome is a congenital genetic disorder in which individuals are born with an extra copy of chromosome 21. The presence of an extra chromosome causes mild to moderate intellectual disability, as well as increased risk for certain medical conditions, such as congenital heart defects, leukemia, various autoimmune disorders, vision and hearing problems, digestive problems, sleep disorders, and others. For reasons no one yet understands, the severity of these conditions varies among individuals.
For more information please refer to the Jerome LeJeune Foundation: http://lejeuneusa.org
For additional information, please refer to the National Down Syndrome Society: http://www.ndss.org