Syndrome, Down: A common chromosome disorder due to an extra chromosome number 21 (trisomy 21). Down syndrome causes mental retardation, a characteristic face, and multiple malformations. Down syndrome is a relatively common birth defect. The chromosome abnormality affects both the physical and intellectual development of the individual. Down syndrome causes cognitive disability, a characteristic facial appearance, and multiple malformations. It is associated with a major risk for heart malformations, a lesser risk of duodenal atresia (part of the small intestines is not developed), and a minor but still significant risk of acute leukemia. Hypothyroidism (low thyroid hormone levels) and celiac disease (gluten intolerance) are also more common in individuals with Down syndrome. The chromosome abnormality that causes Down syndrome is trisomy 21, an extra copy of chromosome number 21.