Physiology

Polydactyly occurs in several species of animals, including cats and humans. In cats, all four feet are affected most of the time.

the offspring will show polydactyly. "The polydactyly trait probably arose as a spontaneous mutation, and a polydactyl kitten from two normal-toed parents may represent a new mutation event," explains Dr. Pflueger. The gene for polydactyly, however, shows variable expressivity (see side-bar), which means that some polydactyl cats have more toes than others. Little has been learned about how the polydactyly gene functions, A Little Bit Extra however. "Up until recently, we saw polydactyly only in household pets Polydactyl cats have extra toes, often on all four feet. Here's rather than as a result of selective what you should know about this charming characteristic. breeding," says Dr. Pflueger, “but because polydactyly is now accepted Larry Houck, a cat owner from Pflueger, MD, director of medical within the Pixie Bob breed Harrisburg, Pennsylvania, has a genetics at the Baystate Medical [recognized by TICA], it may be neutered male cat with more than Center in Springfield, possible to get more information on the normal five toes on his front Massachusetts and chair of the the trait. feet, a condition called polydactyly genetics committee for The The autosomal dominant gene Pd ("poly" meaning multiple and International Cat Association produces a condition that deviates "dactyly" referring to the fingers and (TICA), "and on the back legs, from normal but does not toes). Houck named his cat Rocky, the cat may have just a dew compromise the well-being of the after the movie character, because claw." Although the number of cat. But another unrelated gene, RH, the cat's three extra toes on each extra toes varies from cat to cat, a produces a severely crippling front paw make him look like he's total of 28 toes seems to be the condition called radial hypoplasia wearing boxing gloves. Like Rocky, upper limit. that resembles polydactyly. A cat most polydactyl cats have an Polydactyly can be pre-axial (extra suffering from radial hypoplasia has endearing, even comical quality. thumbs on the inside of the limb) or an unusually small, twisted or absent Polydactyl cats are often referred post-axial (extra fingers on the radius (see sidebar on next page), to as mitten cats or Hemingway cats outside of the limb). In humans, producing the "twisty cat" after the plethora of extra-toed cats polydactyly is most often expressed phenomenon. Although extra toes owned by the writer Ernest post-axially, while in cats, may be present in RH cats, the Hemingway at his home in Key polydactyly is most often pre-axial, normal (Pd) form of polydactyly is West, Florida. In fact, descendants so that a polydactyl cat will appear not harmful. "The gene that of his original polydactyl cats still to have extra thumbs. eliminates or produces a poorly proliferate at the Hemingway home developed radius has nothing to do home, which is now a museum. with the normal form of Polydactyly, also called A Matter of Genes polydactyly," says Dr. Pflueger. hyperdactyly, occurs in several Polydactyly is due to the presence of species of animals, including cats an autosomal dominant gene, Pd and humans. In cats, polydactyly (see sidebar). It is a dominant trait From There to Here can result in any number of extra with nearly 100 percent penetrance. Polydactyly was noted in the digits. "Most of the time, all four "A cat that has the gene will show general cat population as early as feet are affected," says Dr. Solveig the gene," says Dr. Pflueger. If a the late 1800s. Polydactyl cats are polydactyl cat mates with a normal- found all over the United States, toed cat, on average half or more of

but New England and Florida, especially the Keys, seem to have the greatest concentration of them. Polydactyl cats are A Glossary of Terms also found in Great Britain, Southeast Asia and Autosomal dominant gene: a gene carried on a other parts of the world. No one is entirely sure chromosome other than the X or Y (the sex how polydactyl cats came to America, or even if chromosomes) that only has to be inherited from they came here from elsewhere. Some people one parent to be expressed in offspring. A dominant trait inherited from one parent will believe polydactyls are an American phenome- override a corresponding recessive trait npn and made their way to other parts of the contributed by the other parent. globe on sailing vessels. Expressivity: the manner in which an individual shows the effects of a gene. Polydactyly is Other folks believe the reverse. “Are expressed variably from cat to cat. they separate mutation events or the result of human Penetrance: a population genetics term for the commerce?” says Dr. Pflueger. "People bring cats frequency with which a trait is expressed in a with them wherever they go." Whatever its population that carries it. If an individual appears heritage, if you have a polydactyl cat, you may normal despite carrying a gene variation that consider yourself lucky, as did sailors in previous usually produces an abnormality, the trait is said to be non-penetrant in that individual. centuries who traveled with the multitoed Radius: one of the two bones of the forearm in mousers onboard. people or the front leg in cats. Keep your polydactyl kitten healthy, just as you would a normal-toed kitten, with regular veterinary checkups. Trim his toenails regularly to prevent them from becoming ingrown. "If your car has a thumb with an extra toe in between, there may be an increased risk of ingrown toenails,” says Dr. Pflueger.

By Karen Commings