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Polydactyl Cats – Part 2

| November 6, 2010

Polydactyly (six or seven toes) varies from the classic "mitten cat" through to cats which simply have more toes than normal, but no "thumb".Robert O'Rourke's "Paulie" (Paulie-dactyl!) also has extra palm pads on the back feet.

Polydactyly (six or seven toes) varies from the classic "mitten cat" through to cats which simply have more toes than normal, but no "thumb".Robert O'Rourke's "Paulie" (Paulie-dactyl!) also has extra palm pads on the back feet.

THE “BAD” FORM OF POLYDACTYLY

Most expressions of polydactyly are not a handicap to the cat (such as the polydactyl kitten pictured here). The exception is the gene which causes a whole spectrum of effects ranging from extra toes through to radial hypolasia/radial agenesis (the “thalidomide” or “twisty” mutation). This is the gene which causes a condition known as triphalangeal pollex-radial hypoplasia. In mice, there are several gene mutations known to cause this form of polydactylism; unlike conventional “thumb cat” polydactyly, the mutations seem to cause more general disruption of limb formation in an embryo.

Most expressions of polydactyly are not a handicap to the cat

Most expressions of polydactyly are not a handicap to the cat (such as the polydactyl kitten pictured here). The exception is the gene which causes a whole spectrum of effects ranging from extra toes through to radial hypolasia/radial hemimelia/radial agenesis (the "thalidomide" or "twisty" mutation).

This is the “second dominant form of polydactyly” mentioned earlier. With this trait the pollex (thumb) has an extra joint, making it look more like a human finger than the usual rudimentary feline dewclaw. This triphalangeal (three-boned) thumb may be duplicated and sometimes the next digit is also duplicated. A cat with triphalangeal pollices (three-boned thumbs) may produce kittens with hypoplasia (underdevelopment) or aplasia (absence) of the radius, one of the two bones that make up the forearm. Hypoplasia, according to the Cornell Book of Cats, 1997, is “underdevelopment of a given tissue.” So in cats with radial hypoplasia (RH) the radius in the front leg will be underdeveloped or missing.

Because the usual form of polydactyly is so variable in expression, X-rays are needed in order to distinguish between the harmless usual form of polydactyly and the form associated with RH. Some polydactyl cats without dewclaws have 5 or 6 toes all the same length (called “patty” feet). This trait is the one most often associated with RH. However, not all cats with additional same-length toes have the gene for RH. In cats without dew claws or visible thumbs, an X-ray is required to determine whether the radius is deformed in any way – if it is, the cat carries the gene for RH. The gene for RH is variable in expression (possible due to incomplete dominance/heterozygous vs homozygous state), ranging from extra toes to crippling leg deformities, hence the need to determine which non-mitten polydactyls carry the gene for RH and which carry the more usual form of polydactyly.

In a physical examination, the best way to tell the difference is to check for dewclaws. Polydactyl cats without dewclaws are disqualified from shows but may safely produce normal-footed show-quality offspring which can be used in breeding. The cats should be bred with non-polydactyl cats to keep the trait heterozygous as it appears to be the homozygous cats which are affected by RH. A number of genes in cats are less harmful in the heterozygous form, but are either crippling or lethal in the homozygous form e.g. the Manx mutation, the Scottish Fold mutation.

HIND PAWS ONLY POLYDACTYLY

The following photos show Aurora, a hind-paws-only polydactyl Maine Coon kitten bred from non-polydactyl bloodlines by Paul and Natasja in The Netherlands. There are no poly cats within the first six generations and parents, grandparents, about 30 relatives and also related lines are non-polydactyl. Aurora’s 3 brothers have no signs of anything similar.

Aurora, a hind-paws-only polydactyl Maine Coon kitten bred from non-polydactyl bloodlines by Paul and Natasja in The Netherlands

Aurora, a hind-paws-only polydactyl Maine Coon kitten bred from non-polydactyl bloodlines by Paul and Natasja in The Netherlands

Aurora has true dewclaws on both her paws. They form a normal thumb and are complete: footbed, nail and muscles. Usually extra toes only occur on the back feet if the front feet are also polydactyl. Aurora does not have polydactyl front paws. Also unusually, the extra toes on the back paws are true dew claws, placed higher up than ordinary toes. According to Paul “We only discovered it last Friday, when Natasja thought her headache was playing tricks with her. She checked four times if the tail was still at the back, and it still was.”

A similar case has apparently occurred in a Norwegian Forest Cat but there is no further information. Another has turned up in a Manx out of British bloodlines and has not been inherited by any of his offspring (ruling out a dominant gene). Current thoughts are that it is a random mutation, the result of polygenes meeting up or a non-genetic developmental trait. A recessive gene would have shown up in other cats. Aurora will later be test-mated to see if the trait is hereditary.

Stacey Bliss also has a hind paw polydactyl which had an extra toe on the right hind paw and lacked front dew claws. She had previously produced mitten cats, including Stacey’s silver tabby cat Mia, and recently gave birth to a male orange kitten with no front dew-claws, but with extra toes on the hind paws. The kitten’s front feet were at first at an odd angle, being bent down more like a human’s wrist, but as he began to crawl and eventually walk, his feet flattened out. Stacey believes it may have been due to some lack of bones in the feet, but sadly the kitten vanished at the age of 10 weeks before this could be investigated. Mitten-pawed Mia is due to have a planned litter in Spring 2005, after which she will be spayed, and it will be interesting to see what form of polydactyly she passes on.

Aurora has true dewclaws on both her paws. They form a normal thumb and are complete: footbed, nail and muscles.

Aurora has true dewclaws on both her paws. They form a normal thumb and are complete: footbed, nail and muscles.

MINOR PROBLEMS WITH POLYDACTYLY

In general, polydactyly causes no ill-effects in cats. It is certainly not a handicap and is an anomaly (deviation from the norm) rather than a deformity. Although some owners like to have the extra toe removed for cosmetic or safety reasons, cats rarely catch the extra toe(s) on furnishings. In rare cases, nail growth can be affected, but only if the extra toe is incompletely formed and the nail bed is deformed. This can lead to a number of claw problems such as ingrowing claws, overgrown claws or “superclaw syndrome”.

Sometimes a claw develops between the thumb and the rest of the paw as Brian Tinker describes regarding his grey cat “Buddy”. Buddy’s back paws have 5 toes each, 4 normal ones and 1 dew claw. His front paws have between 5 and 7 toes each. The left front paw has 4 normal toes and an opposable thumb that remains underneath the paw and cannot lay in line with the rest of the toes. Between the opposable thumb and the regular toes (i.e. in the angle or crevice) was an additional claw. This was not associated with a toe and simply stuck out from the skin, unfortunately tearing the surrounding skin. Above the thumb is a vestigial toe that is only discernible through handling. On the right front paw, a similar vestigial toe has a segment of extra, jointed bone that can be felt and seen, but has no muscles or tendons controlling it (it can be freely wiggled up and down by the owner). On the right front paw, the thumb lies in line with the rest of the toes, but can also wrap downwards as an opposable digit. One of the claws on the right thumb became deformed and curled around on itself. The claw had died and snapped off easily. The nail-bed has since been removed.

Overgrown claws are not restricted to polydactyl cats, but are more common in polydactyls because the extra toe is often shorter than the regular toes or it points in a slightly different directions. This means that the cat is unable to strop the extra claws on a scratching post. Unless clipped regularly, the claw can become overgrown and embed themselves in the paw pad.

Ingrowing claws grow twisted or crooked instead of growing straight with a smooth downward curve. Ingrown claws can grow into the paw pad and need either more frequent clipping or surgical removal. In the UK, such problem claws are the only time when declawing (of the affected toes only) is permitted. Ingrowing dewclaws can also occur in non-polydactyl cats where the claw grows into side of the foot.

If the claw is set in such a way that it snags on furnishings etc, it can tear and infection can set in. Nail-bed infections or nail bed damage can lead to abnormal claw growth e.g. thickening or twisting. Where the toes are cramped together, the skin between them should be checked for infection as it provides a handy undisturbed crevice for bacteria. In general, the cat’s own cleaning routine should prevent this, but if the toes cannot be spread then its tongue cannot easily clean between them.

If two extra toes are fused together, the nail bed will also be fused. The claw that grows from this dual nail bed can also be fused, leading to something known colloquially as “superclaw syndrome”. The fused claw or “superclaw” is thicker than a regular claw and may twist abnormally. A non-twisting superclaw is not normally painful for the cat, but because the claw is thicker and stronger than usual, it can gouge wood – and flesh – more deeply than a regular claw. If the superclaw grows twisted, there is a danger that it will become ingrown.

Where the extra toe causes repeated problems, it can be removed in a straightforward operation, but this is rarely necessary.

POLYDACTYLY AND THE CAT FANCY

Historically, the polydactyl made up 40% of the original unregistered Maine Coon population. There are claims that the extra toes acted as snowshoes, helping these rugged cats negotiate snowy New England winters. Local folk tales claimed that these cats were fierce hunters who used their oversized paws to catch live fish, even taking fresh fish home to feed their owners! However, breed standards made no allowance for polydactyl Maine Coons and stipulated a normal foot configuration. Because polydactyly in the Maine Coon is due to the autosomal dominant gene, the trait could easily be eliminated by breeding only from non-polydactyl Maine Coons. The trait was deliberately bred out of Maine Coons and only recently have there been attempts to reinstate it.

Selection against polydactyly means the trait seems to have become associated with the term “harmful deformity” in many minds and there have even been postings on Usenet stating, quite erroneously, that “polydactyl cats almost always have some other sort of abnormality”. Many cat registries happily recognise breeds defined by mutations which can have lethal or crippling effects such as spina bifida in the Manx, but refuse to permit polydactyl cats as either breeds or breed variants. Some cat enthusiasts feel that registries are right in refusing to accept polydactyl cats, fearing that breeders would try to produce cats with excessive and disabling numbers of toes on each paw. Since polydactyly doesn’t work in this way and the number of toes appears to be limited, those fears are largely unfounded. In addition, breed standards could be written to define the maximum number of toes in polydactyl breeds to discourage such attempts.

Polydactyly is one of the traits of the PixieBob breed. Early write-ups on this breed suggested that only normal-footed PixieBobs would be accepted for shows. What one registry would not accept, another embraced and as well as the American Polydactyl, PixieBob and Polydactyl Maine Coons there are other polydactyl breeds being developed including the suggestion of a Hemingway Sphynx (a hairless polydactyl cat) and of developing polydactyl Munchkins a (short-legged polydactyl). The Mojave Spotted (formerly Hemingway Spotted) is being developed from Bengal x Polydactyl crosses. Several breeders in Illinois are working with a curl-eared polydactyl cat called Tulips. Tulips were originally developed in the 1990s by crossing American Curls with polydactyls, creating a harlequin patterned semi-longhaired breed. The markings, which can be any colour, are restricted to the head, down the spine, shoulders, hips and tail.

Mojave Spotted  Developed from Bengal x Polydactyl crosses.

Mojave Spotted (formerly Hemingway Spotted): Developed from Bengal x Polydactyl crosses.

In New Zealand, a polydactyl breed called the Clippercat is under development. These are descended from domestic cats that are themselves descended from polydactyls that reached the country on the Clipper Ships between 1850 and 1900.

In Britain, polydactyly is still considered a serious breed fault or defect. According to one breeder, polydactyls can be shown but cannot receive certificates or a first prize in the GCCF. Other sources state that the GCCF absolutely prohibits the showing of polydactyl pedigree cats and that they would be turned away during vetting in (with exceptions for those shown as household pets). A complete bar on the showing of polydactyl cats would be especially unfortunate for owners who wish to show polydactyl household pets and in some registries (FIFe), the prejudice is so great that polydactyl cats were banned from Cat Association shows. They feel that encouraging such abnormalities encourages inbreeding. The blunt statement is that polydactyly is a fault and cats with such defects are not allowed to be shown. This is a totally inconsistent approach since taillessness is also a fault, yet the Manx breed is perpetuated and shown. There are far fewer detrimental side-effects associated with polydactyly than there are with the Manx. The usual argument in these cases is that the Manx is a historical breed even though the polydactyly trait is equally historical. Not to mention that such bodies recognise ultra-typed or extreme Persians whose faces are so compressed that their tear ducts are distorted and their breathing may be compromised. It has to be noted that cat fancies are consistently inconsistent in their approach in such matters!

The GCCF is an extremely restrictive and conservative cat fancy. Its “Standard list of withholding faults – all breeds” (dated 13/10/95) states that certificates and first prizes be withheld for a number of defects considered undesirable in breeding stock and detrimental to individual cats. This includes “Abnormal number of toes – anything other than four toes on each foot and one dew-claw on each foreleg.” This applies only to pedigree breeds; non-pedigrees do not have standards of points. This withholding is usually academic, because cats with “defects” (according the GCCF definition of defect) should not pass the vetting-in stage. The “Rules, section 5, veterinary surgeons” (effective 1 June 1997) has Rule 10 which bars “Exhibits which have been declawed should be rejected (see Section 4 Rule 14) together with polydactyls and cats with folded ears, curly tails or any other abnormality. With the exception of declawing, this does not apply to unregistered non-pedigree exhibits.” In spite of its stance on abnormalities, the GCCF’s double standards means it has not prevented the increasingly abnormal ultra-typing of Persians or Siamese. Even though non-pedigrees are not covered by the GCCF ban on showing polydactyls, it is possible that some judges mark down a polydactyl in the non-pedigree classes because of ingrained views that it is a defect.

The British prejudice against polydactyly may have to change following the importation of PixieBobs into Britain in 2004. In The Netherlands and Belgium, there is a move to restore the polydactyl lines of Maine Coon and the attitudes of the European registries, particularly FIFe, may also have to change. However, in Germany, the prejudice has legal backing as it is forbidden to breed cats (and dogs) that have genetic defects. “Defects” encompasses the harmless anomalies as well as the more harmful mutations. It remains to be seen how the British cat fancies cope with Polydactyl Maine Coons and Pixie-Bobs.

In Germany, the Federal Government erroneously decided polydactyly was a semi-lethal (deferred-lethal) defect and have banned the deliberate breeding of polydactyl cats under their Animal Protection Law. They based the decision on a paper by S & H Willer (“Gene Sites and Alleles of Domestic Cat with Pathological Effects or Side Effects”) which stated, erroneously, that polydactyly is and “autosomal dominant semi-lethal error with modificator effect.” V Schmidt und M. CH Horzinek refer to S & H Willer’s mistake that polydactyly is semi-lethal in the German book “Krankheiten der Katze” (Diseases of the cat).

Having persuaded the American cat fancy that polydactyly was not a deformity, the trait was soon back in the feline geneticists’ spotlight due to its unfortunate association with the Twisty Cat. Twisty Cats have a crippling deformity of the forelegs. They arose spontaneously when a horse-breeder began to breed Poly-Bobs, a type of cat sometimes confused with the PixieBob. Some Poly-Bob litters contained kittens with flipper-like forelegs. The mutation is occasionally found at random in the cat population, but was occurring with greater frequency in Poly-Bob litters. This was the second form of polydactyly and its effects ranged from the Poly-Bob’s simple extra toes through to the Twisty Cat’s vestigial or missing long bones of the leg.

Some breeders and cat lovers have become concerned that the Twisty trait was the hidden downside of polydactyly and that “all polydactyly was bad” despite the fact that perfectly healthy extra-toed cats have been around for hundreds of years. Most forms of polydactyly are no more than a harmless and attractive quirk. In polydactyl cats, it has become important to work out which ones have which gene, so that breeders working with polydactyls do not inadvertantly breed crippled kittens.

In 2006, TICA proposed to clamp down on certain breeding trends, including the creation of new polydactyl breeds created through crossing to other breeds. Their Genetics Committee report stated: “The Committee proposes that TICA does not accept any proposed breeds for Registration Only status that do not exhibit novel mutations. The current mutations would be reserved for currently recognized breeds exclusively. This would end the seemingly endless applications for “munchkinized” new breeds, and then deter the inevitable introduction of “rexed”, “Bob-tailed” and Poly-ed” everything else.”

POLYDACTYLY IN BIG CATS

Polydactyly is not limited to domestic cats. It is also found in big cats, though this is less widely reported for the obvious reason that we do not (normally) share our homes with big cats!

In 1925, The Journal of the Bombay Natural History Society published a photograph of a polydactyl leopard. It had an extra claw-bearing toe on each hind paw. Several years earlier it had published a letter regarding a leopard shot by S Eardley-Wilmot – this creature had an extra claw-bearing toe on each hind paw. In 1946, the same journal published a letter from another big game hunter who had also shot a leopard with extra toes (again fully functional with claws) on the hind feet.

Probably the most interest fact is that the leopards had the extra toes on the hind feet and not the front feet whereas in domestic cats, the hind feet are only affected if the forepaws are affected. This means that a different gene was responsible for hind-foot polydactyly in leopards … and what can occur in a big cat might possible one day appear in domestic cats.

There are unverified reports of polydactyl tigers in China. The tale of a race of unusually large tigers with “thumbs” would appear to be exaggeration, but it is conceivably based on polydactyl individuals. None have been captured or photographed.

GLOSSARY

* A dominant gene is one which shows up when only one copy of that gene is inherited- one for the mother OR one from the father.
* A recessive gene is one which only shows up if the cat inherits two copies of that gene – one from the mother AND one from the father.
* Heterozygous means that the two genes in a pair are different, the cat will not breed true for that trait as some of the offspring inherit the hidden recessive gene.
* Homozygous means that the two genes in a pair are identical and the cat will breed true for that trait.
* Autosomal means the gene is carried on an ordinary paired chromosome, not on the sex-linked X or Y chromosomes.
* Atavism (and atavistic) means the reappearance of an ancestral characteristic after several generations of absence; caused by chance mutation or by recombination of genes.
* Digit means finger or toe.
* A dewclaw is a vestigial (rudimentary) toe or claw which does not touch the ground, it sometimes resembles a thumb (pollex) which is smaller than the other toes.
* The phalanges are the bones inside the fingers and toes.
* Pre-axial means situated in front of the axis of a limb
* The radius is the long bone of the lower forelimb; in humans it is the forearm (elbow to wrist).
* The pollex (plural: pollices) is the thumb.
* The plantar pad is the heel pad of the paw.
* The palmar pad is the palm-pad of the paw.
* The apical cap is the tip of the limb-bud in a developing embryo.

Copyright & Credit:
Sarah Hartwell – MESSYBEAST.COM

Photo copyright and courtesy: Sarah Hartwell

Category: Breeding and Genetics, Feline Health and Care, Feline Resources

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