<P> Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division . There are three forms of nondisjunction: failure of a pair of homologous chromosomes to separate in meiosis I, failure of sister chromatids to separate during meiosis II, and failure of sister chromatids to separate during mitosis . Nondisjunction results in daughter cells with abnormal chromosome numbers (aneuploidy). </P> <P> Calvin Bridges and Thomas Hunt Morgan are credited with discovering nondisjunction in Drosophila melanogaster sex chromosomes in the spring of 1910, while working in the Zoological Laboratory of Columbia University . </P>

Explain the phenomenon of nondisjunction that may occur during gamete meiosis