The polydactyl gene in the Maine Coon breed is the standard benign autosomal dominant gene for polydactyly, Pd. This gene has been identified following the sequencing of the cat genome.

Preaxial polydactyly is caused by ectopic expression of the signalling molecule Sonic Hedgehog (SHH) in the developing limb bud. Mutations in the long-distant, limb-specific cis-regulator for SHH, called the ZRS, are responsible for the ectopic expression which underlies the trait. Populations of domestic cats which manifest extra digits, (including Maine Coons and the celebrated polydactylous Hemingway’s cats), also contain mutations within the ZRS. The polydactylous cats add significantly to the number of mutations previously reported in mouse and human, and to date all are single nucleotide substitutions.

The variant of the Pd gene found in Maine Coons and other North American cats has been called Hw and is slightly different to the Pd genes found in cats from Great Britain (UK1 and UK2). (Ref 3)

The Pd gene is harmless even when homozygous and is not known to be associated with other anomalies. It has complete penetrance.

Sarah Hartwell describes it thus; The form of polydactyly most commonly seen in cats is a simple autosomal (i.e. not linked to gender) dominant trait which does not affect the cat adversely and is not associated with other abnormalities.(ref 2)

Polydactyl is easy to select for or against when breeding Maine Coons. You must have a polydactyl to breed a polydactyl. If you do not have a polydactyl you cannot have one born in a litter.

There are 2 forms of polydactyly; pre-axial and post-axial. Axial refers to the folding of the embryonic limb. The “thumb” side is before the axis (pre-axial) and the “little finger” side is post-axial. In humans it is usually post-axial i.e. an extra little finger, whereas in cats it is normally pre-axial with the extra toe on the thumb side of the foot. The Maine Coon Pd gene is the pre-axial form.

The Pd gene in Maine Coons has a variable expression and can give rise to a varying number of toes with a natural maximum number of 7 toes on each foot. There are different phenotypes associated with the gene. The most common are "mitten paws" and "hamburger paws" and both are pre-axial.

Photos on the right show:
Top: a mitten paw
Bottom: a hamburger paw

In one of the first scientific articles on polydactyly in cats (1949), Charles Danforth (Dept of Anatomy, Stanford University School of Medicine) says: “Although the domestic cat normally has eighteen digits, five on each front foot and four on each hind foot, the occurrence of individuals with more than that number is not uncommon.” (Ref 4)

Earlier studies on guinea pigs had found one form of polydactyly that is lethal when homozygous.

Dr Danforth raised polydactyl cats in his laboratory and reported on 234 normal and polydactyl kittens in 55 litters. His matings reflected a simple dominant trait and he concluded that there was no evidence that the homozygous form was naturally lethal;
the data ...(is) sufficient to show that the type of polydactyly studied behaves as a simple dominant with good penetrance, but variable expression and to indicate that there is no reason to suspect the gene of being lethal when homozygous. (Ref 4)
For some reason it seems that in Germany, officials are of the view that the Pd gene is cats is in fact lethal when homozygous and they reference the Danforth article for evidence of this conclusion. We can only surmise that an error in translation many years ago led them to this erroneous conclusion.

More recently, Dr Solveig Pflueger’s article in the Cat Fanciers Journal 1998 expanded our genetic knowledge of polydactyly. In this article she states; “it does not appear to affect the cat adversely and is not known to be associated with other anomalies”(Ref 5)

Radial Hypoplasia
There has been much scare mongering about the ethics of breeding polydactyl cats caused by the emergence of the “Twisty Cats” in 1998.

Dr Pflueger (MD, PhD, FACMG) is considered one of the world’s leading geneticists. Her 1998 article discussed the usual polydactyl gene (Pd) and the Twisty Cat gene (Tw). After reading her article I had a personal conversation with Dr Pflueger and we discussed the Pd gene and the Twisty Cat gene. Dr Pflueger stated categorically that these 2 genes were quite different genes, that the Tw gene has never been seen in the Maine Coon Cat and never would be unless someone introduces an outcross cat that carries the gene. (Ref 6)

The Tw gene is also a dominant gene with variable expression that gives rise to triphalangeal pollex-radial hypoplasia. An unknown polydactyl cat may in fact have this gene as in its mildest expression it can appear to be normal except for extra digits. However if used for breeding this polydactyl of unknown origin will produce 50% of its offspring with the Tw gene. Some of those will have severe radial hypoplasia (RH). Dr Plueger told me that if a polydactyl cat has been bred from with no incidence of radial hypoplasia then that cat has the usual benign Pd gene and not the Tw gene. (Ref 6)

As the polydactyl gene in Maine Coons has been bred for many generations and can be traced back to foundation stock, it is evident that this gene is not in the Maine Coon. Care must however be taken when introducing new foundation (as is allowed in the breed in the USA) to check that any new polydactyl does not have the Tw gene.

Syndactyly
Another problem falsely associated with polydactyly is the split foot.

Syndactyly (hypodactyly) or split-foot is the opposite of polydactyly. Instead of having additional toes, the cat's forefeet (rarely the hind feet) have two toes giving it the appearance of a crab or lobster claw. In humans, the condition is sometimes known as "lobster-hand". The other digits have either been suppressed altogether or each of the cat's toes is made up of two or more fused digits. A paper by A G Searle (in "Annals of Eugenics" Vol. 17, Part 4, pp. 279-283, 1953) discussed the lobster-claw condition in cats; Searle noted that the anomaly was usually inherited as a dominant, and had suggested that the right side was often more severely affected than the left.

Syndactyly is rarer than polydactyly and is caused by the SP gene.