The normal separation of chromosomes in meiosis I or sister chromatids in meiosis II is termed disjunction. When the separation is not normal, it is called nondisjunction. This results in the production of gametes which have either too many or too few of a particular chromosome, and is a common mechanism for trisomy or monosomy. Nondisjunction can occur in the meiosis I or meiosis II, phases of cellular reproduction, or during mitosis.
This is a cause of several medical conditions in humans, including but not limited to:
- Down Syndrome - trisomy of chromosome 21
- Patau Syndrome - trisomy of chromosome 13
- Edward Syndrome - trisomy of chromosome 18
- Klinefelter Syndrome - extra X chromosomes in males - i.e. XXY, XXXY, XXXXY, etc.
- Turner Syndrome - lacking of one X chromosome in females - i.e. X0
- Triple X syndrome - an extra X chromosome in females
- XYY Syndrome - an extra Y chromosome in males.
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