Nondisjunction

Examples of Nondisjunction in the following topics:

• 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.

• 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.

• 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.

• X-Inactivation

• Sympatric Speciation

  • Aneuploidy results when the gametes have too many or too few chromosomes due to nondisjunction during meiosis.

• Gene Duplications and Divergence

  • Aneuploidy occurs when nondisjunction at a single chromosome results in an abnormal number of chromosomes.