Inbreeding would increase the percentage of polydactyl offspring, but there will always be a few normal-toed kittens in the litter, because of that recessive gene.

What causes human polydactyly?

Polydactyly tends to run in families. It may also result from genetic mutations or environmental causes. The usual treatment is surgery to remove the extra digit.

What causes a cat to be polydactyl?

Polydactyly is caused by a genetic mutation in a dominant gene and usually results in the formation of anywhere between four to seven toes on a kitty's paws. The front paws are most often affected by polydactyly, but it can also occur on the hind paws; it's extremely rare for a cat to have polydactyly on all four paws.