The answer is Meiosis in egg Formation.
Non disjunction is caused by the failure of paired chromosomes to separate during cell division. In meiosis it can occur in meiosis I and meiosis II, if it occurs in meiosis I, it means that at least one pair of homologous chromosomes did not separate. The end result is two cells that have an extra copy of one chromosome and two cells that are missing that chromosome.
