Non disjunction

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Nondisjunction is when chromosomes fail to separate correctly during meiosis leading to the daughter cells containing an unequal distribution of chromosomes. This can have catastrophic effects on the development of an organism.
Nondisjunction or aneuploidy occurs in one of two ways the chromosomes can separate in such a way that both pairs of homologous chromosomes end up in the same gamete. In this case the newly produced cell can contain three pairs or even more pairs of the same chromosome so in a human this would mean that the cell contained 69 chromosomes rather than 23 this is known as a triploidy or tetraploidy. These divisions in meiosis are not as catastrophic as a division that occurs when one gamete has an unpaired chromosome. Monosomy is when one chromosome divides and is unpaired or trisomy where the gamete contains 3 copies of a chromosome one of which is unpaired.

The majority of gametes that are produced with a chromosome abnormality will lead to spontaneous abortion. A gamete that contains a monosomy chromosome is very likely to end up in spontaneous abortion due to having half the amount genes being developed due to one chromosome not being present. However some embryos do develop and can have certain conditions depending on what chromosome incorrectly divided. Since the majority of aneuploidies that actually allow a fetus to develop are trisomy all the conditions are called trisomy n. N being the number of the chromosome that the incorrect division occurred on.
Examples of trisomy conditions are:

Nondisjunction can also occur with the sex chromosomes this division can usually affect an individual if they are lacking a Y or an X. If they have too many copies of the X chromosome there is very little effect as all but one X in a cell is left active the rest are Barr bodies. The same is similar to the Y chromosome, too many copies of the Y chromosome has very little effect on an individual as Y contains very little genes apart from the genes that lead to male development.
The two sex chromosome related conditions are Klinefelter (47XXY) and Turner syndrome (45X) [1].

Nondisjunction occurs more commonly in females than in males as the sperm produced in males have a cell cycle check point mechanism is activated. This leads to programmed cell death where the faulty sperm cell is destroyed by apoptosis [2].


  1. Genetics Analysis of Genes and Genomes, Hartl et al., 2012:268-273.
  2. Essential Cell Biology Alberts et al., 2010:663

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