The genetic change in a population or species over many generations is evolution.
Evolution is the change in the inherited traits of a species or population over several generations. The inherited traits are the expressions of genes that are transmitted from parent to offspring during reproduction. Evolution depends on the process of natural selection and it has resulted to biodiversity of organisms at all level of biological organisation.
A zygote can end up with one fewer chromosome than normal because one of the fertilizing gametes was missing a chromosome. This deletion, called nondisjunction, happened while the gamete was forming.
