Nondisjunction is the failure of chromosome pairs to separate properly during meiosis I or II. To be specific, in meiosis I, homologous chromosomes fail to separate when nondisjunction occurs. In meiosis II or mitosis, the sister chromatids fail to separate. This results in the formation of an aneuploid cell. ie, a cell with an imbalance of chromosomes, unlike a normal cell. During meiosis, if nondisjunction occurs, it takes place during the anaphase when the chromosomes are separated on the spindle fibers and directed to opposite poles.
Down's syndrome is a condition also known as trisomy 21, which means there are three copies of the 21st chromosome unlike the normal two copies. Ultimately there are 47 chromosomes in the individual rather than the 46 chromosomes. Trisomy 21 is the result of a meiotic nondisjunction event that took place when either the maternal gamete or the paternal gamete was formed. 88% of the cases owe to flawed maternal gamete.
Chromosome 21 houses around 200 to 250 genes. When there are 3 copies of this chromosome, there is an absolute over-expression of these genes, that contribute to the symptoms that are expressed in a patient affected with Down's syndrome.
Non-disjunction is when homologous chromosomes or sister chromotids do not separate properly during meiosis. This can occur during either anaphase I or anaphase II when the chromosomes are being separated by the spindle.
If the homologous chromosomes do not separate correctly during say Anaphsase I, then one daughter cell will end of with three copies of a chromosome and the other daughter cell will end of with one copie of the chromosome. One example of this happening is in downs syndrome. One parent had a nondisjunction occur which led to their sperm/egg having two copies of chromosome 21. When the sperm/egg with two copies fertilized it receievd another copy of chromosome 21 for a total of 3. The three copies of chromosome 21 are what result in downs syndrome.