Atavisms are the sudden reappearance of a trait in an organism that has not been seen in its recent ancestors. They are caused by the expression of a recessive gene that has been inherited from a distant ancestor.

Atavisms can occur for a number of reasons. One possibility is that the gene for the atavistic trait has been passed down through generations of the organism's ancestors, but it has only expressed itself in the current organism due to a change in environmental conditions or a random mutation. Another possibility is that the atavistic trait is actually a dominant gene, but it has only been expressed in the current organism because the other copy of the gene (the recessive allele) has been mutated or deleted.

Atavisms can be interesting and fascinating, but they can also be a sign of a genetic disorder. Some atavisms, such as the reappearance of a tailbone in humans, are relatively harmless. However, other atavisms, such as the reappearance of a jawbone in snakes, can be more serious and even life-threatening.

Here are some examples of atavisms:

* Humans with tails

* Snakes with legs

* Chickens with teeth

* Pigs with stripes

Atavisms provide evidence for the theory of evolution. They show that organisms can inherit traits from their distant ancestors, even if those traits have been hidden for many generations.