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Non-disjunction

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Non-disjunction

Non-disjunction ("not coming apart") is the failure of chromosome pairs to separate properly during meiosis stage 1 or stage 2, specifically in the anaphase. 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.

In the event that an aneuploidic gamete is fertilized, a number of syndromes might result. The only known survivable monosomy is Turner syndrome, where the individual is monosomic for the X chromosome. Examples of trisomies include Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Patau syndrome (trisomy 13).

Nondisjunction types

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

           2n          
        /      \        
     2(n+1)    2(n-1)
    /   \      /   \
  n+1   n+1  n-1   n-1  
         2n          
      /      \        
    2n       2n
    / \      / \      
   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)

Though a person of any age can have nondisjunction occur during meiosis, the chances tend to increase with age.

References

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

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