Fail to separate or nondisjunction is the failure of one, some, or all chromosomes to separate during meiosis. Nondisjunction occurs in meiosis I or II It can cause a change in chromosome quantity within genital cell.
|Non-disjunction homologous chromosomes in meiosis I|
|Nondisjunction chromatid in meiosis II|
The change in chromosome quantity depends on the quantity of non-disjunctive chromosome. If nondisjunction occurs only in a certain number of chromosomes, for instance, number 5, then, the change of chromosome quantity only occurs in number 5. On the other hand, if non disjunction occurs in all chromosome numbers, then, all chromosome numbers (from no I to 23) will have a change in their quantity. As a result, the formed gamete may contain twice as many as the normal quantity and the other gamete may not contain chromosome at all. If an organism having one set of even normal chromosomes (2n, 4n, 6n) undergoes nondisjunction, then, the new offsprings produced will have odd chromosome set (n, 3n,5n)1. The new offsprings usually cannot produce cell (sterile).
Source: Essential Biology, 2006