HOW POLYDACTYLY IS INHERITED
Updated: Sep 12
Polydactyly is often described as a dominant trait. In Maine Coons, that is correct, but it is only the beginning of the genetic story.
Polydactyly is one of those characteristics that appears simple until inheritance is examined closely. A cat has additional digits, one of its parents is polydactyl, and the trait is described as dominant. From there, it is easy to assume that the genetics are straightforward.
They are not quite that simple.
Research in Maine Coons supports an autosomal dominant pattern of inheritance, but also reveals variable expressivity, incomplete penetrance in Hw associated cats and more than one genetic background for the phenotype within the breed. Those details help explain why apparently similar cats can produce different outcomes, why a visibly standard pawed cat carrying Hw can transmit the variant, and why a familiar percentage should never be mistaken for a guarantee.
The inheritance is real.
The outcome is not mechanically predictable.
How polydactyly is inherited through dominant inheritance
Autosomal means that the relevant genetic factor is not carried on a sex chromosome.
Dominant means that one copy of a causative variant can be sufficient for the associated characteristic to appear.
For some forms of feline polydactyly, researchers have identified variants within a regulatory sequence known as ZRS. This sequence controls activity of Sonic Hedgehog, usually abbreviated to SHH, an important signalling molecule during limb development.
Changes within ZRS can alter the normal pattern of SHH activity in the developing limb and result in the formation of additional digits.
Three feline ZRS variants have been described in the scientific literature: UK1, UK2 and Hw. The original molecular work demonstrated that these variants could alter regulatory activity associated with limb development.
When Maine Coons were studied specifically, Hw was found in some polydactyl cats.
It was not found in all of them.
That single fact already tells us that the expression the polydactyl gene is too simple for the breed as a whole.
Why people talk about fifty percent
The familiar fifty percent figure comes from a particular genetic situation.
If one parent carries one copy of a dominant variant and one ordinary copy, while the other parent carries two ordinary copies at that same locus, each kitten has a one in two probability of inheriting the variant from the carrier parent.
That does not mean exactly half of a litter must inherit it.
Each kitten represents a separate inheritance event.
A litter of four is not biologically required to contain two kittens that inherited the variant and two that did not. A litter may happen to fall close to the expected proportion, or it may look very different.
The percentage describes probability.
It does not describe a quota.
There is another important qualification.
Where penetrance is incomplete, inheriting the variant does not necessarily guarantee visible expression of the phenotype. Fifty percent in this model therefore describes the probability of transmitting the variant, not a promise that fifty percent of the kittens will visibly display polydactyly.
That distinction is particularly important in small litters, where chance alone can produce results that look very different from a theoretical expectation.
Genotype and phenotype are not always identical
Genotype describes the genetic information an animal carries.
Phenotype describes what can actually be observed.
With many dominant characteristics, those two things appear to correspond neatly. A cat carries the relevant variant and visibly expresses the characteristic.
The Maine Coon research showed that this is not always the case with Hw associated polydactyly.
In a study of 75 Maine Coons, 48 were polydactyl and 27 were not.
Two males that appeared nonpolydactyl were found to carry one copy of Hw. Their standard paw phenotype was confirmed radiographically. Both subsequently produced polydactyl offspring when mated with nonpolydactyl females from breeding lines in which polydactyly had not been reported.
Those males carried and transmitted Hw despite not visibly expressing polydactyly themselves.
That is incomplete penetrance.
Incomplete penetrance
Penetrance describes whether a genotype produces the phenotype associated with it.
If penetrance is complete, every individual carrying the relevant genotype expresses the characteristic.
If penetrance is incomplete, some carriers do not.
The two Hw carrying males in the Maine Coon study provide direct evidence that incomplete penetrance can occur with this variant.
For breeders, the practical significance is considerable.
A standard pawed Maine Coon is not, by appearance alone, proof that Hw is absent where that variant is segregating in the line.
That statement should not be extended beyond the evidence.
The study demonstrated incomplete penetrance for Hw associated polydactyly. By contrast, the inheritance pattern in the Canadian line best fitted an autosomal dominant model with complete penetrance. Because the molecular cause in that line was not identified, however, the researchers could not confirm its penetrance at the molecular level.
The distinction matters because Maine Coon polydactyly does not appear to arise from one identical genetic cause in every breeding population.
Variable expressivity
Penetrance asks whether the trait appears.
Expressivity asks how it appears.
Among 18 polydactyl Maine Coons carrying one copy of Hw in the 2020 study, 13 had polydactyly confined to the forepaws and five had all four paws affected.
The study also contained one Maine Coon carrying two copies of Hw whose additional digits were confined to the hind paws.
This is variable expressivity.
The same associated variant can produce different visible outcomes.
That is why the paws of a parent cannot be treated as a precise template for the paws of its offspring.
A cat with dramatic polydactyly does not necessarily produce kittens with equally dramatic feet.
A less extensive phenotype does not, by itself, demonstrate a weaker genetic effect. The study showed substantial variation in expression among cats carrying Hw.
The phenotype describes how the characteristic developed in that individual animal.
It does not measure the strength of the inherited variant.
Toe count is not inherited as a fixed number
This point is often misunderstood.
A cat with seven toes on a paw does not transmit an instruction that says seven to its kittens.
What is inherited is genetic information affecting the patterning of the developing limb.
The eventual number of digits, which paws are affected and how those digits are arranged are aspects of phenotypic expression.
This is why related cats can share an inherited cause of polydactyly while displaying strikingly different feet.
Toe count is therefore not a simple hereditary number that can be reproduced with mathematical precision.
Why Maine Coon polydactyly cannot be reduced to Hw
This is one of the most important findings in the modern research.
The 2020 study included 48 polydactyl Maine Coons.
Nineteen carried Hw.
Twenty nine did not carry Hw, UK1 or UK2.
Those 29 cats belonged to the Canadian breeding line. Researchers investigated additional regulatory regions involved in limb development but did not identify the causative variant responsible for polydactyly in that group.
The conclusion was that Maine Coon polydactyly is genetically heterogeneous.
In practical terms, the same broad visible characteristic can arise from more than one genetic background within the breed.
There is therefore no scientifically sound basis for treating every polydactyl Maine Coon as though it carries one universal mutation.
Some carry Hw.
Others are demonstrably polydactyl without carrying Hw, UK1 or UK2.
In the Canadian line examined, the precise molecular cause remained unidentified.
Why bloodline matters
The genetic study did more than separate cats into polydactyl and nonpolydactyl groups.
It also examined their relationships and population structure.
The Hw carrying polydactyl Maine Coons came from breeding lines founded with American polydactyl cats. The 29 polydactyl cats without any of the three known ZRS variants belonged to the Canadian line.
Population analysis also demonstrated genetic differentiation between the Hw and Hw free groups.
This makes line history genuinely useful.
A pedigree may show polydactyly recurring through a particular branch.
It may reveal a standard pawed ancestor that subsequently produced polydactyl offspring.
It may show that a particular phenotype has persisted through generations.
What a pedigree does not reveal automatically is the complete molecular genotype of every cat within it.
Pedigree knowledge is therefore valuable.
It is not a substitute for genetic evidence where an appropriate validated test exists.
Can two standard pawed Maine Coons produce a polydactyl kitten?
Yes, under specific genetic circumstances.
The two visibly nonpolydactyl Hw carrying males documented in the Maine Coon study subsequently produced polydactyl kittens.
The explanation was not the spontaneous appearance of the trait.
Hw was already present.
It simply was not visibly expressed in the fathers.
That is precisely what incomplete penetrance means.
This finding should not be interpreted to mean that polydactyly can simply appear in any mating between two standard pawed Maine Coons. In the documented cases, the Hw variant was already present in the fathers despite the absence of a visible polydactyl phenotype.
The lesson is narrower, and more important.
Appearance alone does not always reveal genotype.
Can two polydactyl parents produce a standard pawed kitten?
Under a simple dominant model, yes.
If both parents each carry one copy of the same causative dominant variant and one ordinary copy, each parent can transmit either copy to a kitten.
A kitten that inherits the ordinary copy from both parents would not inherit that particular dominant variant.
That is a straightforward Mendelian expectation under a defined genetic model.
It should not be turned into a universal prediction for every mating between two polydactyl Maine Coons.
Different breeding lines do not necessarily share the same identified molecular cause, and penetrance cannot be assumed to be identical across all forms of the phenotype.
The model is useful.
The genetic background still matters.
Homozygosity should not be overinterpreted
A homozygous cat carries two copies of the same variant.
The Maine Coon study identified one cat that was homozygous for Hw.
That individual was polydactyl, with additional digits confined to the hind paws.
One animal cannot establish broad conclusions about homozygosity across a breed.
Interestingly, the earlier work by Lettice and colleagues also identified a homozygous Hw cat outside the Maine Coon study population that did not display a more severe phenotype.
The evidence therefore does not justify claiming that two copies of Hw necessarily produce a more extensive phenotype, a particular paw configuration, a health problem or a superior breeding outcome.
Scientific restraint matters just as much as scientific knowledge.
What DNA testing can and cannot tell us
A genetic test can identify only the variant or variants it is designed to detect.
If a laboratory tests for Hw, the result tells us whether Hw was detected in the sample.
It does not automatically tell us whether another genetic cause of polydactyly is present.
The Canadian Maine Coons in the 2020 study make this particularly clear.
They were visibly polydactyl, yet the 29 cats examined did not carry Hw, UK1 or UK2.
Their phenotype was real.
The known tested variants simply did not explain it.
A negative result for one known variant therefore means that that particular variant was not identified.
It does not mean that there is no genetic basis for the polydactyly.
Those are very different conclusions.
What a breeder can state with confidence
The published Maine Coon research supports several conclusions.
Polydactyly in the populations examined followed an autosomal dominant pattern of inheritance.
Incomplete penetrance was demonstrated in Maine Coons carrying Hw.
Expression varied considerably among cats carrying the same known variant.
Hw explained polydactyly in only part of the population studied.
A separate Canadian line contained polydactyl Maine Coons without Hw, UK1 or UK2, and the precise molecular cause in that line remained unidentified.
The researchers therefore concluded that Maine Coon polydactyly is genetically heterogeneous.
Beyond those conclusions, the wording should become more careful.
Some outcomes can be expressed as probabilities.
Some remain hypotheses.
Some questions are still unanswered.
That is not a weakness in the science.
It is the boundary of what the available evidence allows us to say.
What this means for a prospective owner
For a prospective owner, the genetics help explain why polydactyl Maine Coons can look so different from one another, even within the same family.
A kitten with more dramatic paws is not necessarily genetically better.
A sibling with fewer additional digits is not an inferior Maine Coon.
The visible paw is the individual expression of an inherited developmental characteristic.
Toe count should therefore never become a shortcut for judging the quality of a kitten or the quality of the breeding behind it.
Polydactyly is one characteristic within the complete cat.
Health, temperament, structure, pedigree and the way a kitten has been raised remain separate considerations.
Inheritance is not prediction
The statement polydactyly is dominant is useful.
It is also incomplete.
Inheritance tells us how genetic information can pass from one generation to the next.
Probability tells us what outcomes may be expected under defined conditions.
Neither can promise the appearance of an individual litter.
A fifty percent transmission probability does not guarantee half a litter.
A dramatic paw does not predict an identical paw in the offspring.
A standard paw does not always prove that Hw is absent where Hw is segregating.
A negative result for one known variant does not exclude another genetic cause.
The more closely Maine Coon polydactyly is examined, the more important pedigree, line history, penetrance, expressivity and genetic background become.
For breeders, that is the real lesson.
The objective is not to force the biology into the neatest possible percentage.
It is to understand exactly what the evidence allows us to say, and to recognise when the correct answer is that the genetics are not yet fully resolved.
Buba Ayers
Founder & Breeder, Tycoon Manor
Sources and further reading
Hamelin A, Conchou F, Fusellier M, Duchenij B, Vieira I, Filhol E, et al. Genetic heterogeneity of polydactyly in Maine Coon cats. Journal of Feline Medicine and Surgery. 2020;22(12):1103 to 1113. DOI 10.1177/1098612X20905061.This is the principal source for the Maine Coon specific inheritance data, including the 75 cat cohort, Hw findings, incomplete penetrance, variable expressivity and genetic heterogeneity.
Lettice LA, Hill AE, Devenney PS, Hill RE. Point mutations in a distant Sonic Hedgehog cis regulator generate a variable regulatory output responsible for preaxial polydactyly. Human Molecular Genetics. 2008;17(7):978 to 985. DOI 10.1093/hmg/ddm370.The foundational molecular study identifying feline ZRS variants and demonstrating how altered regulation of SHH can produce polydactyly.
Hamelin A, Begon D, Conchou F, Fusellier M, Abitbol M. Clinical characterisation of polydactyly in Maine Coon cats. Journal of Feline Medicine and Surgery. 2017;19(4):382 to 393. DOI 10.1177/1098612X16628920.A complementary Maine Coon study documenting the breadth of visible and skeletal expression of the phenotype across different breeding lines.
