Definition: Nondisjunction occurs when the chromosomes in the cells do not separate correctly when they form the eggs or sperm. Nondisjunction causes one of the new egg or sperm cells to have two copies of a certain chromosome, while the other egg or sperm cell has no copies of that chromosome. This event occurs more commonly in egg cells as a woman ages, and it can cause chromosomal abnormalities such as Down syndrome, where the fertilized egg has two copies of chromosome 21.