Often extra or missing chromosomes are caused by a process
known as nondisjuction, occurring during cellular meiosis (the process that
makes reproductive cells – eggs and sperm). During meiosis, the chromosome
pairs line up along the middle of the cell in metaphase and separate during
anaphase. Nondisjunction occurs when a chromosome pair does not separate
properly, resulting in a reproductive cell with two copies of that particular
chromosome (instead of the typical one) and another reproductive cell to not
have any copies of that chromosome. This situation will result in sperm/egg
cells with 22 and 24 chromosomes rather than the typical 23.
(http://www.hudsonalpha.org/education/kits/disorder-detectives/chromosome-abnormalities)
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