Causes of Down Syndrome

Causes of Down Syndrome

Trisomy 21  is caused by a meiotic nondisjunction event. With nondisjunction, a gamete (i.e., a sperm or egg cell) is produced with an extra copy of chromosome 21; the gamete thus has 24 chromosomes. When combined with a normal gamete from the other parent, the embryo now has 47 chromosomes, with three copies of chromosome 21. Trisomy 21 is the cause of approximately 95% of observed Down syndromes, with 88% coming from nondisjunction in the maternal gamete and 8% coming from nondisjunction in the paternal gamete. The actual Down syndrome "critical region" encompasses chromosome bands. Until the middle of the 20th century, the cause of Down syndrome remained unknown. However, the presence in all races, the association with older maternal age, and the rarity of recurrence had been noticed. Standard medical texts assumed it was caused by a combination of inheritable factors that had not been identified. Other theories focused on injuries sustained during birth.

Causes of Down Syndrome

Causes of Down Syndrome

Causes of Down Syndrome

Causes of Down Syndrome

Causes of Down Syndrome

Causes of Down Syndrome

Causes of Down Syndrome

Causes of Down Syndrome

Causes of Down Syndrome