Biochemical Fault

Humans normally have 46 chromosomes in each cell, divided into 23 pairs. There are two copies of chromosome 21. One copy inherited from each parent, form one of the pairs. The fault that causes Down Syndrome is a random error in cell division that results in having an extra copy of chromosome 21. This is the only fault that causes the conditions. It is not known why this error occurs, and it has not been linked to any environmental or behavioural factors, either before or during pregnancy, but the risk is markedly increased with the age of the mother. At age 25, the risk is about 1 in 1,250 births; at age 40, it is 1 in 100.