Some researchers have claimed that five is the magic number when it comes to digits - they say that any more and the animal becomes literally all fingers and thumbs. This was based on the assumption that the genetic mutation that produces the extra toes is also responsible for deformities and that possession of more than five digits is a counter-survival trait. But in the case of cats, having extra toes seems to have no ill effects on them at all. In fact, the inner toes on the front feet can often be opposable in these cats, meaning they can have almost human dexterity. I've never seen one of these cats open a jar or tin though.
Cats with this mutation are known as polydactyls (from the Latin for 'many digits'), or as Hemingway cats. Tourists visiting Ernest Hemingway's house (Nobel prize winner for literature) will find around 60 polydactyl cats strolling about, thought to be descended from his first cat. Polydactyly is thought to result from a dominant gene that is passed through generations of cats (dominant means that only one of a pair of genes needs to be mutated for the animal to be physically affected, recessive means that two copies of a gene are required). The gene affects the tissue forming at the end of the limb in the developing embryo. If the end of the developing limb is larger than normal, extra toes develop.
The thing is, I once had a polydactyl cat and I know that none of his brothers and sisters had extra toes. If it were truly a dominant trait you would expect it to be roughly 50-50 in one liter. So I did some research and found that there's actually some debate over whether it is caused by a dominant gene after all. It could instead be an incomplete dominant. Normally with a dominant trait, the cat either has it or doesn't. With incomplete dominance, there are levels depending on whether the cat has one or two genes for the trait. 'Normal' cats from the same family as a polydactyl could also be affected but with such small extra toes that no one notices.
Polydactyly is not limited to domestic cats. It is also found
in big cats, though this is less widely due to the problems with
getting close enough to a big cat to count its toes without losing
your head.
