Nondisjunction is seen in the autosomes where it produces a trisomic condition for the chromosome pair in question. For example, in the diagram at the right, a gamete with a non-disjoined 21st chromosome pair is fertilized by a normal germ cell to produce a zygote with the normal diploid chromosome number except for the 21st pair which is trisomic. In humans, this condition produces Down's syndrome and becomes a significant possibility when women have children toward the end of their child-bearing years. It seems that the normal mechanisms that assure a segregation of the 21st pair homologues into different gametes breaks down and the incidence of Down's infants rises.
