Genetics Report -Dewclaws Information, Spring Issue 2007

Genetics Report: Breeding for Dewclaws (or how I lost an important breed characteristic in two easy steps)
Sharon K. Schaefer

In our breed dewclaws must be present on all four legs. When I purchased my first Icelandic I searched for a puppy with not just rear dewclaws but double rear dewclaws. When her six grand puppies were born here last year, two had no dewclaws at all on one hindleg. Two others had singles on both hindlegs, and on one of these they were quite weak. I didn’t truly understand what a good or poor double dewclaw looked like.

By failing to focus hard on the conformation of good dewclaws on every leg and individual differences between both legs and dogs I had lost a critical characteristic of the Icelandic Sheepdog.

Given the low numbers of Icelandics that most of us have met in real life and the small gene pool of our breed in Canada I hope that this article will prevent other breeders from unknowingly (note: I may still have chosen to do this mating) reproducing this event.

I would especially like to acknowledge the efforts and information shared by Monika Karlsdottir in the articles, drawings, and charts on her website - I had access to these before this mating but difficulty in downloading and my own lack of knowledge about their importance prevented me from persevering to study them more thoroughly ahead of time.

Front single dewclaws are normal for canids and polydactyly is common in dogs. Removal of front and/or rear dewclaws at birth is a routine procedure for most North American purebred dogs. Rear dewclaws are essentially unknown in wild canids and have been taken to mean hybridization with dogs had occurred.


This theory has recently been supported by molecular comparisons of some Italian wolves, a few of which had rear dewclaws. More than six FCI breeds require rear dewclaws, usually double, and several more breed standards either prefer or allow double dewclaws. As a generalization, the trait is preferred in breeds working on snowy or uneven ground requiring extra grip. Some breed standards such as for Briards and Norwegian Lundehunds include detailed drawings of both varying correct toe structures and foot placement as well as unacceptable ones.


Dr. Sheila Schmutz indicated last year that the genetics of dewclaw structures was unknown at that time and I was unable to locate any genetic studies on the inheritance of dewclaws in dogs other than Monika’s. This is not a trait likely to warrant scientific research. As a multifactorial trait, and since Briard pups are still being born without the correct bony attachments, we can anticipate that despite our informed efforts lack of correct dewclaws will continue to be one source of pet puppies in our breed!


In people, there are several forms of isolated polydactyly (occurring on its own and not as part of a larger syndrome) which are caused by an autosomal dominant gene with varying degrees of both penetrance and expressivity. In these cases a family may have two normal appearing (one of whom carries the gene but without expression) parents, so there is a 50% chance of the children carrying the gene. One or more children may exhibit different expressions of polydactyly: a normal appearing extra digit, a stub, or two extra digits. One chromosome has been mapped and they are working at comparing phenotypic occurrences with genotype. In cats(early Maine Coon and Ernest Hemingway’s cats), polydactyly also occurs as an autosomal dominant gene with about 50% of the population exhibiting the trait.


A similar genetic basis would fit the occurrence of rear dewclaws and double dewclaws in our dogs. Monika Karlsdottir in her detailed articles and charts proposes that as many as 12 genes (12 different loci or gene locations on the chromosomes) would fit the phenotypic expression occurring in Icelandic Sheepdogs. Monika refers to this as a threshold trait where a dog needs a certain number of the dominant genes present in order to develop rear dewclaws.


For development of double rear dewclaws the number of dominant genes required is larger to get over the threshold. An Icelandic without rear dewclaws still quite probably carries some of the dominant genes required, just not enough of them. An Icelandic with good strong rear double dewclaws has enough of the dominant genes present but we still have no way of determining their genotype. As we can’t test for the presence of these genes we have to guess based on our knowledge of their parents’ dewclaws, grandparents, siblings, aunts, uncles, etc. As they produce puppies we learn more about the probable genetics of each sire and dam but it is still educated guesswork! As this is one of the notable features of Icelandics and one of the few disqualifying factors for showing the guesswork becomes important. The greater the number of loci (maybe 12!) determining a trait, the more continuous the variability: strong low claws forming a broader foot, strong low claws, strong not so low claws still with bony attachments, strong low single claw, weak but double claws, one strong and one weak claw forming the double, single attachment with two claws, weaker single attachment with two claws, or it could be millimetres of difference in location on the leg. Then there is the other hindleg!!!



For breeding purposes, I now classify my dogs based on the attachment to the leg. The photos with this article illustrate the dewclaws of some offspring and the parents - both classified single left rear dewclaw for breeding purposes.



There are also some wonderful pictures of dewclaws on Tofra Puppy pages. www.tofradogs.com/English/Articles.htm
http://article.pubs.nrc-cnrc.gc.ca (Paolo Ciucci, Vittorio Lucchini, Luigi Boitani, and Ettore Randi)
www.briard.com
http://cat.inist.fr
www.cfainc.org
http://en.wikipedia.org
www.hmc.psu.edu
http://library.thinkquest.org


Doctors’ Schmutz Genetics Seminar – Part 2 will be in the next Newsletter but I will again refer you to the Colour Genetics site authored by Sheila and which is very frequently updated as new research knowledge becomes available worldwide. There are also links to other genetic sites of interest.
http://homepage.usask.ca/~schmutz/dogcolors.html