Atavisms occur when a characteristic appears in an organism that was not present in its immediate ancestors but was common in more distant ancestors. This is usually due to a hidden gene that has been reactivated.

Here is an example of how atavism works:

- A grandparent has bushy, prehensile tails.

- Both parents are missing tails, as is the child.

- However, the child develops tails during puberty.

This may be because the child inherited genes for a tail that were dormant in both parents. When the child triggered their puberty genes, the tail genes became active and the child developed a tail.