Definitions for "Non-Disjunction"
Due to failure in pairing of homologous chromosomes in meiosis, the two members of one pair migrate to the same pole, giving rise to unbalanced gametes, one of which contain both homologous chromosomes, and the other none (most frequent in sex chromosomes). The non-disjunction event is much more frequent in maternal meiosis I. This may be due to the fact that in a mature woman, oocytes have been held in the ovary for a very long time at prophase I of meiosis from before her birth to shortly before ovulation of the oocyte in question.
Failure of disjunction or separation of homologous chromosomes or chromatids in mitosis or meiosis, resulting in too many chromosomes in some daughter cells and too few in others. Examples: In meiosis, both members of a pair of chromosomes go to one pole so that the other pole does not receive either of them; in mitosis, both sister chromatids go to the same pole.
The failure of a pair of chromosomes to separate during cell division (mitosis), resulting in an abnormal number of chromosomes in the resulting new cells. Non-disjunction of chromosome 21 is the mechanism underlying most cases of Down syndrome.