A normal cat has 18 toes: four toes and a dewclaw on each of the front paws and four toes on each of the back paws. Some cats, known as polydactyls, have extra toes. If you have one of these cats, she possesses a rather common trait among cats.
Genetics, Not Bad Breeding
Because extra toes are a bit of an abnormality, there's a mindset that there's "bad" breeding involved with polydactyl cats. However, it's all in the genes and it doesn't matter if the gene comes from mom or dad. Polydactyl cats have the dominant gene, Pd. If one parent of the litter has the Pd gene, about 40 to 50 percent of the kittens will inherit the gene. It doesn't mean they'll be polydactyl, but they will have the gene for extra toes. If two parents have the Pd gene, it's possible there will be normal 18-toed kittens in the litter because the Pd parents can also carry a recessive gene for normal toes.
Although you don't come across a six- or seven-toed cat every day, it's not particularly rare. There's no information that estimates the number of polydactyl cats in the world, or even in the United States, but there are some interesting trends. The most famous population of extra-toed cats is at the Ernest Hemingway Home and Museum, where about half of the approximately 80 to 100 cats in residence have more than 18 toes. New England is reported to have a high incidence of polydactyls, particularly in Boston, where the cats are referred to as "Boston thumb cats." Though fairly common in England, they're all but non-existent in the rest of Europe.
Maine Coon Cats
Polydactylism is a genetic trait, not a breed. However, it's a more common trait in Maine coon cats. In the past, coon cats had an incidence of about a 40 percent rate of polydactylism. The theory is the extra toes helped the cats walk in the snowy landscape, effectively acting as a snow shoe; however, this is considered to be more of a fun myth than a fact.
A Little Unusual, Not Rare
As with polydactylism in general, it's impossible to say what the incidence of seven-toed cats is compared to five-toed cats or six-toed cats. There was a report from the 1960s of an English tomcat fathering polydactyl kittens. According to the story each generation of cats added an additional toe. Of course, there's no confirmation of the story, and if true, it's only anecdotal evidence. The bottom line is that a seven-toed cat is unusual, but not rare. However, it's Mittens' personality and relationship with you that makes her rare and wonderful, no matter how many toes she has.