A process called is an error in meiosis in which chromosomes fail to separate and a gamete with too many or too few copies of a particular chromosome can be produced.

The process you're referring to is called "nondisjunction". This occurs during meiosis, the process of cell division that produces gametes (sex cells).

Here are the steps:

1. In a normal meiosis, chromosomes duplicate and create sister chromatids - identical copies of themselves. These sister chromatids are supposed to separate during meiosis so that each resulting gamete has the correct number of chromosomes.

2. Nondisjunction occurs when these sister chromatids fail to separate properly. This can happen in either the first or second division of meiosis.

3. If nondisjunction happens in the first meiotic division, both sister chromatids of a particular chromosome go into the same daughter cell.

4. If nondisjunction happens in the second meiotic division, the sister chromatids fail to separate and both go into the same gamete.

5. As a result of nondisjunction, one gamete ends up with two copies of the particular chromosome, and the other gamete ends up with no copies.

6. If these gametes are involved in fertilization, the resulting zygote will have either three copies of the chromosome (a condition known as trisomy) or only one copy (a condition known as monosomy). Both of these conditions can lead to serious genetic disorders. For example, Down syndrome is caused by trisomy of chromosome 21.