Before meiosis occurs, a diploid cell (2n) in the ovaries or testes will undergo a special type of reduction division resulting in four haploid sex cells. This process is called meiosis. First, the DNA in the gonad cell is replicated.
In meiosis I, homologous chromosome pairs line up at the metaphase plate. Next, members of each homologous pair separate and move to opposite poles during anaphase I. By telophase I, the chromosomes are at opposite poles and a nucleus reforms around them. Next, the cytoplasm divides producing two cells. These two cells will now enter meiosis II. The resulting 4 daughter cells are haploid (n) with half of the chromosomes of the parent cell and can serve as gametes or sex cells.
However, if nondisjunction occurs in meiosis I, the homologous chromosomes fail to separate. This results in all four gametes containing an abnormal number of chromosomes-too many or too few.
Aneuploidy describes the variation in chromosome number due to errors of meiosis. The gametes can be n-1 missing a chromosome or n+1 with an additional chromosome.
I have included a link to an animation showing meiosis and non-disjunction.