Meiosis is a type of cell division by which chromosome number is reduced to half. It occurs during gamete formation. However, during anaphase I, a pair of chromosome fails to separate or during anaphase II sister chromatids fail to separate during anaphase II of meiosis. Both the cases results into nondisjunction of chromosomes. Nondisjunction forms abnormal number of chromosomes in the gametes.
Nondisjunction causes human disorders in autosomes from chromosomes 1-22. The examples are Down syndrome or trisomy 21, an extra copy of chromosome 21. People with Down syndrome have 47 chromosome instead 46.
