Failure of separation of sister chromoatids or homologues during mitosis or meiosis.
The consequence of nondisjunction are aneuploidies. When nondisjunction occurs during meiosis then the result is gametes that either have too few or too many chromosomes, most typically one too many (trisomies) or one too few (monosomies).
Only one human monosomy is viable, that of the X chromosome, resulting in what is known as Turner syndrome. Most autosomal trisomies are not viable at birth or through early childhood. The prominent exception is trisomy 21, or Down syndrome.
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