Explanation of How the Egg Cell Acquired an Extra Chromosome (Chromosome 21)

Normally, in the oocyte stage of an egg cell, there are two sets of chromosomes that exist together.

In the normal cell, one of the chromosomes leaves the cell.

In nondisjunction in mitosis I, both chromosomes remain in the cell (they do not separate from one another).

When the sperm finally inserts its chromatid into the cell, the cell looses a chromatid. This ultimately results in a cell with three copies of a chromatid (which results in the genetic disorder Down Syndrome).