Nondisjunction

Nondisjunction is the failure of chromosome pairs or sister chromatids to separate properly during cell division. Somatic cells, i.e. cells not related to sexual reproduction, like muscle cells, adipocytes, epithelial cells, blood cells, etc, divide by mitosis. Germ line cells divide using a specialized process, called meiosis, to give rise to gametes, eggs (oocytes) or sperm (spermatozoa). Both forms of cell division require the ordered distribution (segregation) of paired chromosomes or sister chromatids. Nondisjunction ("not coming apart") is an erroneous chromosomal segregation that results in an abnormal number of chromosomes (aneuploidy). There are three forms of nondisjunction involving segregation during the cell cycle: failure of a pair or homologous chromosomes to separate in meiosis I, or the failure of sister chromatids to separate during meiosis II or mitosis.
Calvin Bridges and Thomas Hunt Morgan are credited with discovering nondisjunction in...