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Nondisjunction ("not coming apart") 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. Such a cell is said to be aneuploid. Loss of a single chromosome (2n-1), in which the daughter cell(s) with the defect will have one chromosome missing from one of its pairs, is referred to as a monosomy. Gaining a single chromosome, in which the daughter cell(s) with the defect will have one chromosome in addition to its pairs is referred to as a trisomy.

The following diagram shows the two possible types of nondisjunction in meiosis:

           2n          
        /     \        
      n+1      n-1
     /   \    /   \
  n+1   n+1  n-1  n-1  
         2n
      /     \         
    n        n
   / \     /   \      
  n   n  n+1   n-1    
Schematic of nondisjunction in meiosis I. Duplicated chromosomes in diploid cell (2n).

All gametes are affected by nondisjunction in meiosis I. Two gametes have a single extra chromosome; two gametes are missing a single chromosome.

Schematic of nondisjunction in meiosis II. Duplicated chromosomes in diploid cell (2n).

Half of the gametes are affected by nondisjunction in meiosis II. One gamete has a single extra chromosome; one gamete is missing a single chromosome.

"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)

Disorders Resulting from Nondisjunction

Examples of human disorders resulting from nondisjunction in gametes include:

Autosomal chromosome aneuploids

There are no viable autosomal monosomies and no other viable autosomal trisomies; they are all embryonic lethal (i.e. cause miscarriage in early pregnancy).

Sex chromosome aneuploids

Cancer cells often are monosomic or trisomic for certain chromosomes. This is evidence of a nondisjunction event during mitosis in that cell's lineage.

External links

References

Snustad, D.P., Simmons, M.J.(2006). Principles of Genetics (4th ed.). Hoboken: John Wiley & Sons, Inc.

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