nondisjunction
(noun)
the failure of chromosome pairs to separate properly during meiosis
Examples of nondisjunction in the following topics:
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Disorders in Chromosome Number
- Aneuploidy, an abnormal number of chromosomes in a cell, is caused by nondisjunction, or the failure of chromosomes to separate at meiosis.
- Misaligned or incomplete synapsis, or a dysfunction of the spindle apparatus that facilitates chromosome migration, can cause nondisjunction.
- The risk of nondisjunction occurring increases with the age of the parents.
- Nondisjunction can occur during either meiosis I or II, with differing results.
- Nondisjunction may occur during meiosis I or meiosis II.
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Down Syndrome
- Trisomy 21 is caused by a meiotic nondisjunction event.
- With nondisjunction, a gamete (i.e., a sperm or egg cell) is produced with an extra copy of chromosome 21; the gamete thus has 24 chromosomes.
- Trisomy 21 is the cause of approximately 95% of observed Down syndromes, with 88% coming from nondisjunction in the maternal gamete and 8% coming from nondisjunction in the paternal gamete.
- Trisomy 21 is usually caused by nondisjunction in the gametes prior to conception, and all cells in the body are affected.
- This can occur in one of two ways: a nondisjunction event during an early cell division in a normal embryo leads to a fraction of the cells with trisomy 21, or a Down syndrome embryo undergoes nondisjunction and some of the cells in the embryo revert to the normal chromosomal arrangement.
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Fertilization
- Nondisjunction during the completion of meiosis or problems with early cell division in the zygote to blastula stages can lead to problems with implantation and pregnancy failure.
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X-Inactivation
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Sympatric Speciation
- Aneuploidy results when the gametes have too many or too few chromosomes due to nondisjunction during meiosis.
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Gene Duplications and Divergence
- Aneuploidy occurs when nondisjunction at a single chromosome results in an abnormal number of chromosomes.