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Nondisjunction

Disorders of chromosome number include the duplication or loss of entire chromosomes, as well as changes in the number of complete sets of chromosomes. They are caused by nondisjunction, which occurs when pairs of homologous chromosomes or sister chromatids fail to separate during meiosis.

Two cases of nondisjunction are presented. For the first case, a cell is shown during metaphase I, where two pairs of red and blue X shaped structures are aligned in the centre. The cell moves to anaphase I, where the nondisjunction takes place, resulting in one pair of red and blue X shaped structures pulled towards the same pole. As a consequence, at the end of meiosis two, four cells are formed, two of them having two blue and one red rod shaped structures named n plus one and two of them having only one blue rod shaped structure, named n minus one. For the second case, the X shaped structures are correctly separated at meiosis one, but during anaphase two in meiosis two, one blue X shaped structure in one of the cells is being pulled towards one pole, instead of being separated into half, and each half going to one pole of the cell. As a result, at the end of meiosis two, four cells are formed, two of them having two rod shaped structures, named n, one of them having one blue X shaped structure and one blue rod shaped structure, named n plus one, and one of them having one blue rod shaped structure, named n minus one.

Figure 1. Nondisjunction.

Nondisjunction can occur during either meiosis I or II, with different results:

  • Nondisjunction Meiosis I

If nondisjunction occurs in meiosis I, both chromosomes of a homologous pair migrate to the same pole, leaving one daughter cell without any chromosome. The cell then goes through meiosis II normally. The products are 4 gametes: two of them have one complete set of chromosomes plus one extra chromosome (n+1), and the other two gametes lack a chromosome (n-1).

  • Nondisjunction Meiosis II

If nondisjunction occurs in meiosis II, both sister chromatids of a chromosome migrate to the same pole of the cell. Only one chromosome separates abnormally. The products are 4 gametes: two gametes are normal, one has extra chromosome (n+1) and one lacking a chromosome (n-1).