Nondisjunction

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'Nondisjunction' is the failure of chromosome pairs to separate properly during cell division. This could arise from a failure of homologous chromosomes to separate in meiosis I, or the failure of sister chromatids to separate during meiosis II or mitosis. The result of this error is a cell with an imbalance of chromosomes. When a single chromosome is lost (2n-1), it is called a monosomy, in which the daughter cell(s) with the defect will have one chromosome missing from one of its pairs. When a chromosome is gained, it is called trisomy, in which the daughter cell(s) with the defect will have one chromosome in addition to its pairs.

Examples of nondisjunction: Down's Syndrome, Triple-X syndrome, Klinefelter's Syndrome, Turner's Syndrome

The following diagram shows the two possible types of nondisjunction in meiosis: N.B. "n" denotes a cell with a single copy of each chromosome (haploid cell); 2n denotes a cell with two copies of each chromosome (diploid cell)

          2n(4c)          Duplicated chromosomes in diploid cell.  DNA content (c-value) is given in brackets.
       /         \        Schematic of nondisjunction in meiosis I.
     n+1         n-1
    (2c)        (2c) 
    /   \       /   \
  n+1   n+1   n-1    n-1  All gametes are affected by nondisjunction in meiosis I. 
 (~c)  (~c)  (~c)   (~c)  Two gametes have a single extra chromosome; two gametes are missing a single chromosome.         

        2n(4c)           Duplicated chromosomes in diploid cell.  DNA content (c-value) is given in brackets.
      /        \         Schematic of nondisjunction in Meiosis II.
     n          n
   (2c)       (2c)
   /  \       /   \      
  n    n    n+1   n-1    Half of the gametes are affected by nondisjunction in meiosis II.
 (c)  (c)  (~c)  (~c)    One gamete has a single extra chromosome; one gamete is missing a single chromosome.
                          

N.B. Normal c-values will change slightly in aneuploidal cells; values are given here to illustrate the major changes in cellular DNA content.

External links

• Human Genetics
• Genetics Home Reference
• Other Syndromesde:Non-Disjunction

nl:Non-disjunctie sv:Nondisjunction