How does nondisjunction relate to birth defects?

In the normal process of meiosis, chromosomes pair at the metaphase plate and subsequently separate and migrate to opposite poles. When nondisjunction occurs, the chromosomes do not properly separate. Instead, both members of one homologous chromosome pair migrate to the same pole. As a result, half the gametes will have an extra chromosome, and half will be missing a chromosome. Gametes with missing or with extra chromosomes are usually sterile. However, sometimes, certain chromosome imbalances are fertile. These almost always lead to genetic defects.

For example, Down syndrome occurs when an egg or sperm with an extra number 21 chromosome fuses with a normal gamete. The result is a zygote with three copies of chromosome 21.