Disorders of chromosome number包括整个chromoso的重复或损失mes, as well as changes in the number of complete sets of chromosomes. They are caused bynondisjunction, which occurs when pairs of homologous chromosomes or sister chromatids fail to separate duringmeiosis.

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).