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Down syndrome is a disorder of abnormal number of chromosomes. Down syndrome is usually caused by an extra copy of chromosome 21 (trisomy 21). Overview of children with Down syndrome include growth retardation, mental retardation with varying degrees; craniofacial abnormalities, including upward slanting eyelids, epicanthic fold (extra skin folds at the medial corner of the eye), flat face and small ears, heart defects, and hipotonia. These individuals also showed increased incidence of leukemia, infections, thyroid dysfunction, and premature aging. In addition, nearly all showed signs of Alzheimer's disease after the age of 35 years. In 95% of cases, this syndrome is caused by trisomy 21 due to meiotic nondisjunction, and in 75% of these cases, nondiscjunction occurs during the formation of the oocyte. The incidence of Down syndrome is about 1 of the 2000 conceptus for women younger than 25 years. This risk increases with maternal age from 1 in 300 at age 35 years to 1 in 100 at age 40 years.
At approximately 4% of cases of Down syndrome, occurs non-balanced translocation between chromosome 21 and chromosome 13, 14, or 15. The remaining 1% is caused by mosaikisme caused by mitotic non-disjunction. These individuals have a normal number of chromosomes of cells and some cells are aneuploid.