Numerous assertions have been made over the years that polydactylism is a breeding defect that will produce extreme expressions and severe maladies in the Maine Coon. This subject has triggered many a heated debate when poly breeders counter to defend the fitness of their cats. Scientific studies and empirical experience now provide abundant evidence: the polydactyl trait in the Maine Coon is an innocent variance that poses no threat to the well-being of the cat.
Note: Over time, more and more is discovered about the specific mechanisms of the genetics of polydactyly. In 2008, Lettice, et al, reported that the polydactyl trait in cats is not due to a gene mutation or a (Pd) gene but rather to a single point mutation of the regulatory element responsible for expression of the gene that determines digit formation (see specifics below). While their terminology may be dated, earlier studies and research remains valuable for the observations they provide.
1. In 1947 The Danforth Studies of a largely in-bred population from two DLH Dams produced 254 poly kittens which were observed and logged: “The trait is not related to sex, and no evidence is found that its gene is lethal”. Danforth found no evidence of split foot or radial hypoplasia (also called radial hemimelia) in his studies. (Polydactyl Cats – Part 1 Copyright 2001-2009) http://www.messybeast.com/poly-cats.html
2. “The form of polydactyly most often seen in cats is the result of a simple autosomal dominant trait. It does not appear to affect the cat adversely and is not known to be associated with other anomalies.” (Polydactyly and Related Traits – Dr. Solveig Pflueger, Fall 1998) http://mcpolydactyl.com/rationale/4-Pflueger_Traits.pdf
3. The autosomal dominant gene Pd produces a condition that deviates from normal but does not compromise the well-being of the cat. There is another unrelated gene, RH, that produces a severely crippling condition called radial hypoplasia that resembles polydactyly. A cat suffering from radial hypoplasia has an unusually small, twisted or absent radius producing the “twisty cat” phenomenon. Although extra toes may be present in RH cats, the normal (Pd) form of polydactyly is not harmful. “The gene that eliminates or produces a poorly developed radius has nothing to do with the normal form of polydactyly,” says Dr. Pflueger. (‘A little bit extra’ by Karen Commings, Catwatch Feb’06, interview with Dr. S. Pfleuger) http://mcpolydactyl.com/rationale/1-Catwatch_Feb06.pdf
4. Dr. Leslie Lyons (University of California, Davis) has worked with Maine Coon breeders for years, collecting DNA samples to identify the polydactyl gene. Her determination coupled with other studies states the following: “The Pd gene is absolutely harmless even when homozygous and has nothing in common with the RH gene.” (Dr. Lyons in Moscow) http://mcpolydactyl.com/rationale/5-Dr_Lyons_Moscow.pdf
“The fact that remains is the gene is variable in expression regardless of breeding combinations. It is not lethal or even different in expression in its homozygous form, as is the case in some other expressions of a dominant gene.” (Dr. Lyons Speaks before the WCC in Arnhem, The Netherlands) http://mcpolydactyl.com/rationale/6-Dr_Lyons_Arnhem.pdf
5. 2008, Laura A. Lettice, Alison E. Hill, Paul S. Devenney and Robert E. Hill from the MRC-Human Genetics Unit, Western General Hospital in Edinburgh U.K. This paper investigated polydactylism in the feline world, in an attempt to expand knowledge of the currently known genes that produce polydactyl expression in various species, including humans, mice and cats. The feline study identified three mutations of polydactyly that were similar but that had slightly different expressions. It was found that the polydactyl trait in cats is not due to a gene mutation or a (Pd) gene but to a single point mutation in a sonic hedgehog cis-regulator element known as ZRS. Sonic hedgehog (Shh) is the gene that determines digit formation, and is regulated by a non-coding ZPA regulator sequence (ZRS) which is a distant cis-element in cat’s DNA. This information not only revealed a previously unknown mechanism of control, but brought the total number of identified genetic expressions of polydactylism to thirteen, three of which were specifically associated with cats. Although noted in the study that polydactylism can be a problem in other species, no issues were found in the feline world. Subjects included pedigreed Maine Coons, as well as pedigreed Pixie Bobs and British Cats. All three mutations were benign expressions of polydactyly. The study concluded that: “Analysis of polydactylous cats identified three new mutations…we submit that this type of polydactyly has no further detrimental effect on the cat’s health”. (Human Molecular Genetics, 2008, Vol. 17, No. 7 978–985) http://mcpolydactyl.com/rationale/7-Molecular_Genetics.pdf
6. 2011, Alexia Hamelin: La Polydactylie du Maine Coon – École Nationale Vétérinaire d’Alfort. Ms. Hamelin published as her dissertation the first analysis worldwide on pre-axial polydactyly combining genetic and phenotypic data. Approximately 100 polydactyl cats were examined, with the majority (86) being Maine Coons. Sixty cats were analyzed genetically and sixty by X-ray. Sixteen Maine Coons of American lineage demonstrated the mutation type first reported by Lettice et al in 2008. In forty-one Maine Coons of Canadian lineage, however, the specific mutation could not be identified but was not part of the cis-regulatory mechanism described previously. This study supported earlier suppositions that polydactyly in Maine Coons is an autosomal-dominant trait with complete penetration and high variability. Empirical tests of reproduction, body height, peri-natal, mortality and malformation showed no significant statistical differences between polydactyl and non-polydactyl Maine Coons. This data leads to no concerns for the morphology or health of the cats. The mutation is a purely aesthetic modification passed on in dominant autosomal mode and with variable expression or outcome. (La Polydactylie de Maine Coon) http://theses.vet-alfort.fr/telecharger.php?id=1334
7. 2013, Axel Lange, Hans L. Nemeschkal, Gerd B. Müller – Department of Theoretical Biology, University of Vienna. Research on the genetics of the formation of the polydactyl paw continues with this technical publication describing the molecular basis of the formation of the polydactyl limb. The different phenotypic polydactylous patterns of the Maine Coon are described in detail (fore and hind limb) and the paper presents a mathematical/statistical model of how a point mutation can result in new digits, using threshold effects in cell states as an explanation. http://link.springer.com/article/10.1007/s11692-013-9267-y
8. 2016, Alexia Hamelin, Dominique Begon, Fabrice Conchou, Marion Fusellier and Marie Abitbol: Clinical characterisation of polydactyly in Maine Coon cats. Polydactyly has been reported in a number of vertebrate species, including the domestic cat. It is a common characteristic in some breeding lines of the Maine Coon. The aim of this study was to assess the limb phenotype of polydactyl cats using physical and radiographic examinations. Physical examination and radiography were used to characterize the polydactyly phenotype in a cohort of 70 Maine Coon cats, including 48 polydactyl cats from four different breeding lines from Europe, Canada and the USA. The phenotypic expression of polydactyly showed great variability, not only in digit number and conformation, but also in the structure of the carpus and tarsus. Comparison of the size of the radius in polydactyl and non-polydactyl 3-month-old kittens and adult females did not reveal any difference between polydactyl and non-polydactyl cats. Conclusions: polydactyly in Maine Coon cats is characterized by broad phenotypic diversity. Polydactyly not only affects digit number and conformation, but also carpus and tarsus conformation, with no apparent deleterious consequence on feline welfare.
In 2006 the database service, PolyTrak, was established to examine expressions of polydactylism in the Maine Coon breed. Although not scientific from the standpoint of controlled breeding and observations in a sterile lab, the PolyTrak studies have involved a large number of pedigreed Maine Coon kittens and adults in “real-world” settings in the seven-plus years it has functioned. This observational study continues to compare the scientific studies with data tracking from catteries and pet-homes throughout the world.
With over 1300 Maine Coons, no noted detrimental effects from a genetic standpoint have been observed, lending credence to published studies and observations (source: PolyTrak 2014 – Litter & Breeder Tracking).