Cat body type genetic mutations

Cats, like all living organisms, occasionally have mutations that affect their body type. Sometimes, these changes in body type are striking enough that humans select for and perpetuate them. This is not always in the best interests of the cat, as many of these mutations are harmful; some are lethal in their homozygous form.

This page gives a selection of cat body type mutant alleles and the associated mutations with a brief description.

Tail Types
Jb = Japanese bobtail gene (dominant with incomplete penetrance). Cats heterozygous for this gene have abnormal tails, but unlike the Manx cat there are no associated skeletal disorders and the gene is not associated with lethality.

M = Manx gene (dominant). Cats with the homozygous genotype (MM) die before birth, and stillborn kittens show gross abnormalities of the central nervous system. Cats with the heterozygous genotype (Mm) show severely shortened tail length, ranging from taillessness to a partial, stumpy tail. Some Manx cats die before 12 months old and exhibit skeletal and organ defects. People have suggested that the Manx gene, because it was discovered in naturally occurring populations of cats, is a gene conferring some kind of selective advantage to the cats. However, the most likely reason for this gene's natural occurrence is genetic drift in an island population.

There are numerous other bobtail types in the cat population, most of which are identical to the Japanese Bobtail or the variably expressed Manx mutation. However, some may be novel mutations that have not been investigated.

There are numerous types of curly-tailed cats whose tails loop over the back or form tight corkscrews. One such mutation has been developed into the American Ringtail, but others have been regarded as curiosities and not perpetuated. The gene(s) responsible have not been fully investigated.

Limbs
Mk = Munchkin gene (dominant). Cats heterozygous for this gene (Mkmk) have shortened legs, but are not disabled. They have a ferret-like gait. The homozygous form (MkMk) may be lethal as litter sizes are smaller than average. Although there was initial concern that Munchkin-type cats would have impaired mobility or spinal problems, this was based on comparison with dog breeds and proved to be unfounded due to the cat's more flexible spine. The mutation has occurred naturally in many locations and has also been perpetuated in feral cats without human intervention (Robinson's Genetics for Cat Breeders).

The mutation has proven not to be achondroplasia, but is most likely to be either hypochodroplasia or pseudochondroplasia which affect the long bones of the leg while leaving other bodily proportions, especially the head, unchanged.

Paws
Sh = Split Foot (Syndactyly). A dominant gene that reduces the number of toes resulting in a "lobster-claw" appearance. This is considered an undesirable mutation.

Polydactyl (extra-toed) cats. There are probably many genes, both dominant and recessive, that cause polydactyly in cats. Most cases of polydactyly in cats are perfectly harmless.

Pd = Thumb-cat polydactyly gene. The Pd gene (dominant with incomplete penetrance) causes the benign, pre-axial form of polydactyly where one or more extra toes occur near the dew claw. Often, the dew claw is converted into a thumb. There are occasional problems such as fused claws or claws facing in the wrong direction, but generally, this form of polydactyly is harmless.

On the other hand, the "hamburger-feet" polydactyly gene is associated with gene for radial hypoplasia (RH). The 1995 European Convention for the Protection of Pet Animals considers RH an impairing condition. In a scandal in the late 1990s, an experimental breeder in Texas tried to perpetuate this deformity as the "Twisty Cat" breed. Mild RH can cause the post-axial form of polydactyly - enlarged paws, extra three-jointed toes on the outer, little-toe side of the paws, and no thumb. X-rays can determine the structure of the extra toes and whether the cat has the gene for RH. Cats with the gene for RH should never be bred. Cats with severe RH have unusually short front legs. They move like a ferret and they tend to sit like a squirrel or kangaroo and are colloquially known as squittens. In some RH cats, the forelegs are twisted with the long bones either severely shortened or absent. All polydactyl cats are banned from German cat shows, possibly because of confusion with the impairing form of polydactyly associated with RH.

Polydactyl cats are relatively common in southwest Britain, Norway, Sweden, and the eastern coast of the USA and Canada, and some parts of Asia. Sailors thought they were lucky. There were, and are, many myths surrounding polydactyl cats:
 * That they are superior mousers and ratters,
 * That they have better balance on ships in stormy weather,
 * That their paws are natural snowshoes,
 * That their opposable thumbs (in the thumb-cat form of polydactyly) give them a survival advantage.

Ear Types
Cu = American curl gene (dominant). Cats with this gene have ears that start out normal, but gradually curl backwards. So far, no harmful defects have been associated with this gene.

Fd = Scottish fold gene (dominant with incomplete penetrance). Cats with this gene have ears that curl forward. There are different degrees of folding, and more genes may be involved in the expression of the Fd gene. This gene is associated with bone and cartilage defects such as thickened tail and swollen feet. The homozygous form (FdFd) is probably lethal.

Australian Curl - a curl-eared mutation occurred in a female stray cat in Australia, but was not inherited by her offspring. When the original cat became ill, necessitating spaying, it was impossible to test-mate her sons back to her to identify a possibly recessive curled-ear mutation.

Some information reproduced with permission from Messybeast.com