Meiotic Nondisjunction: Term referring to the failure of 2 similar chromosomes to separate correctly so both go to a single cell and none to the other. This occurs during the course of the 1st meiotic split. Also refers to the failure of 2 chromatids of a chromosome separating correctly so both chromatids go to one cell and none to the other. This occurs during mitosis or in the course of the 2nd meiotic separation. Due to this, one cell will have 2 chromosomes or 2 chromatids and the other will have none. When this occurs during meiosis, an aneuploidy individual – for instance, child with Down syndrome – might develop following fertilization.

It can also be an innate term that refers to an occasion that takes place during cell division, in which due to an accident of genetics causes the sperm or egg cell to have 24 chromosomes, instead of the normal 23. This too can lead to a child with Down syndrome.