During cell divisions, the chromosomes separate, with one of each of the sister chromatids going to opposite poles of the cell. Therefore, each new cell has the correct number of chromosomes. Cell division in which sister chromatids fail to separate properly, which does happen occasionally, is called nondisjunction.
If nondisjunction occurs during meiosis I or meiosis II, the resulting gametes will not have the correct number of chromosomes. When one of these gametes fertilizes another gamete, the resulting offspring will not have the correct number of chromosomes.
Nondisjunction can result in extra copies of a certain chromosome or only one copy of a particular chromosome in the offspring. Having a set of three chromosomes of one kind is called trisomy. Having only one of a particular type of chromosome is called monosomy.
Nondisjunction can occur in any organism in which gametes are produces through meiosis. In humans, alterations of chromosome numbers are associated with serious human disorders, which are often fatal.
