nondisjunction

Nondisjunction is a genetic error where homologous chromosomes or sister chromatids fail to separate properly during cell division (meiosis or mitosis). This results in daughter cells receiving an incorrect number of chromosomes, with one cell getting both and the other getting none.1

This can lead to numerical abnormalities in chromosomes, affecting both autosomes and sex chromosomes. For example, nondisjunction during gamete formation can result in eggs or sperm with extra or missing chromosomes. This is a primary cause of conditions like Down syndrome, where an individual has an extra copy of chromosome 21. It can also lead to sex chromosome abnormalities, such as Turner syndrome (a single X chromosome) or Klinefelter syndrome (an extra X chromosome in males). The risk of nondisjunction increases with maternal age.234