What is nondisjunction?

Nondisjunction is the failure of chromosomes to seperate properly during meiosis in which, division of chromosomes did not happen efficiently during either meiosis 1 or meiosis 2, most likely occured during meiosis 1. And for it to happen during either one of the type of meiosis, it leads to different effects. If it happened during the division in meiosis 1, the cell that has been divided are all not normal meanwhile, if it involved in meiosis 2, the tendency to get not normal progeny is 50%.

Nondisjunction is the failure of one or more pairs of homologous chromosomes or sister chromatids to separate normally during nuclear division, usually resulting in an abnormal distribution of chromosomes in the daughter nuclei.