Answer:
The process of reproduction is a pretty complex one, and there are several steps that are taken that, if taken wrong, can lead to abnormalities. The most important thing to know is that all humans have two sets of 23 chromosomes, one from each of their parents. The correct process of egg and sperm formation is meiosis, and it will ensure that from the originating cell, the gametes (or eggs) formed, will have the total amount of chromosomes needed.
During this process of meiosis, the chromosomes should be separated and evenly divided. But first, DNA replication must take place. This means that first two homologous chromosomes, and are created so that all new cells will have the same DNA information. This happens in Meiosis I, Anaphase I. On the second phase of meiosis, in its anaphase (meiosis II, anaphase II) a new division will happen, but this time, of sister chromatids. This separation process is called disjunction.
So, in answer to the question above, we have that:
1. Non-disjunction: is when the two pairs of homologous chromosomes (in meiosis I, anaphase I), or the two sister chromatids (in meiosis II, anaphase II), do not separate, or parts remain with the other.
2. This happens during two phases of Meiosis: Meiosis I, anaphase I and Meiosis II, anaphase II.
3. The result can be two cells with extra copies of chromosomes, and the other missing that extra pair of chromosomes and in the case of nondisjunction in anaphase II, the results can be: two normal haploid cells, and the other two will, one have an extra chromosome, and the other, be totally missing it.
4. Some examples of these situations, when female and male gametes unite to form a baby, would be: Trisomies (extra copies of chromosomes) and Monosomies (absent normal numbers of chromosomes).
