More on Mutations
Mutations can be caused in many ways. Spontaneous mutations arise as a result of normal cellular processes. For example, DNA polymerase occasionally makes errors during DNA replication. Conversely, other mutations are caused by physical or chemical mutagens.
One powerful physical mutagen is ultraviolet light, which is one of the components of sunlight. When an animal stays out in the sun too long, the ultraviolet light causes the formation of thymine dimers in the DNA of its skin cells. Thymine dimers are chemical crosslinks between two adjacent thymines. As you can see in this animation, these dimers cause the DNA chain to distort and can cause DNA polymerase to make lots of errors. These errors can ultimately lead to skin cancer.
Animals with pale coats, such as white and grey horses, have very high incidences of skin and eye cancer, possibly because they do not have dark pigments to help protect their skin from ultraviolet light.