Summary Eggs and sperm are formed following meiosis. This is a two step process resulting in the replication and segregation of each chromosome pair, and the formation of four gametes from the original cell. Normally, the eggs and sperm contain a set of each of the 23 chromosomes. At fertilization, the total chromosome count of 46 chromosomes is restored. Following mitosis, all somatic cells will have 46 chromosomes.
Nondisjunction occurs when a pair of chromosomes fails to separate during meiosis.
This results in the formation of eggs or sperm with too many or too few chromosomes. When these chromosomally abnormal gametes are fertilized the resulting fetus is most often miscarried. Infants with chromosome abnormalities who do survive have physical abnormalities and intellectual delays. The incidence of nondisjunction increases with maternal age. For instance, women in their early 20's have a 1 in 500 chance of having a child with a chromosome abnormality, increasing in the mid 40's to 1 in 21.