The Nondisjunction reference article from the English Wikipedia on 24-Jul-2004
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Nondisjunction

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In biology, nondisjunction is the failure of a chromosome to split correctly during meiosis.

This results in the production of gametes which have either more or less of the usual amount of genetic material, and is a common mechanism for trisomy or monosomy.

Nondisjunction can occur in the meiosis I or meiosis II phases of cellular reproduction.

This is a cause of several medical conditions, including: