Nondisjunction

Meiosis is a very tightly regulated process, and a whole series of control mechanisms (constituting a number of "checkpoints") exist to ensure that everything proceeds in the correct manner. If an error should occur during the process, it is usually corrected. Nondisjunction is the result of a mistake at the level of chromosome segregation, which involves the spindle fibers. In normal meiosis, there is a mechanism that monitors the correct formation of the spindle fibers, the correct attachment of the chromosomes to the spindle fibers, and the correct segregation of chromosomes. Collectively, this is referred to as the spindle checkpoint. Failure of this checkpoint to function correctly results in nondisjunction of chromosomes.

Nondisjunction is known to occur more frequently in the cells of older individuals. This is illustrated by the fact that older women are more likely to give birth to children affected by an aneuploid condition than are younger women. For instance, the risk of a twenty-year-old mother giving birth to a child with Down syndrome is about one in two thousand, compared to an approximate one in thirty risk in the case of a woman of age forty-five. The precise reason for this is not entirely certain, but a simple explanation could be that the older a cell is, the more loosely controlled are the processes occurring within that cell. This would mean that an older cell undergoing meiosis would be more likely than a younger one to ignore the constraints of the spindle checkpoint and hence give rise to aneuploid cells.