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Nondisjunction is the failure of chromosome pairs to separate properly during meiosis or mitosis. The result of this error is a cell with an imbalance of chromosomes. When a chromosome is lost, it is called a Monosomy, in which the gamete with the defect will have one or more chromosomes missing from one of its pairs. When a chromosome is gained, it is called a Trisomy, in the case of a human cell it will have a set of 24 chromosomes instead of the usual 23.

A diagram of 2 possible occurrences of nondisjunction in meiosis: (n means 1 copy of each chromosome [haploid cell]) (2n means 2 copies of each chromosome)[diploid cell])

             4n         (duplicated chromosomes in diploid cell)
       /         \       (non-disjunction occurs here)
     3n           1n
   /     \      /     \
n+1  n+1    n-1   n-1   (Two gametes have one extra chromosome and the other 2 are missing a chromosome)
          4n          (duplicated chromosomes in diploid cell)
      /        \
    2n        2n
   /  \    /     \      (non-disjunction occurs here)
  n   n    n+1  n-1     (2 Gametes are normal, but on the right side, one gamete has an extra chromosome, and the other is missing one)

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