nondisjunction is the failure of homologous chromosome 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. 600px|thumb|1. meiosis i 2. meiosis ii 3. fertilization 4. zygote the left image at the blue arrow is nondisjunction taking place during meiosis ii. the right image at the green arrow is nondisjunction taking place during meiosis i. nondisjunction is when chromosomes fail to separate normally resulting in a gain or loss of chromosomes. ==types== in general, nondisjunction can occur in any form of cell division that involves ordered distribution of chromosomal material. higher animals have three distinct forms of such cell divisions: meiosis i and meiosis ii are specialized forms of cell division occurring during generation of gametes (eggs and sperm) for sexual reproduction, mitosis is the form of cell division used by all other cells of the body. === meiosis i === [[file:trisomy due to nondisjunction in maternal meiosis 1.png|600px | thumb| nondisjunction in meiosis i trisomy is most frequently caused by nondisjunction in maternal meiosis i. the sequence of events of this type of nondisjunction is schematically contrasted with normal meiosis. in metaphase i, the two homologous chromosomes assemble in the so-called bivalent, which enables inter-chromosomal exchange of genetic material through homologous recombinat
Genetics