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'Nondisjunction' is the failure of chromosome pairs to separate properly during meiosis or mitosis. This could be 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 chromosomes 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.

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 I.
 (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.

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