Nondisjunction
DNA replication duplicates every strand of DNA so that there are two copies of every chromosome. These (attached) copies, called sister chromatids, undergo two rounds of chromosomal divisions ( Meiosis I and II). A total of four haploid cells are now present. Homologous chromosomes or sister chromatids are pulled apart in a process called disjunction. However, chromosomes may at times fail to separate properly. This is called nondisjunction.