What is "nondisjunction"

n. (context genetics English) The failure of chromosome pairs to separate properly during meiosis

n. meiosis in which there is a failure of paired homologous chromosomes to separate; results in an abnormal number of chromosomes in the daughter cells

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).

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.