In this installment of Inbred Mistakes, I take on a highly educated and well respected man of letters who is also an experienced breeder: Dr. Bruce M Cattanach BSc PhD DSc FRS. You might know him as the breeder who introduced Corgi into the UK Boxer breed to achieve a short tail without the need to dock. His resume speaks for itself:
B.Sc (Hons) in Agriculture, 1955; Ph.D in Mutagenesis and Mammalian Genetics, 1959; D.Sc in Mutagenesis and Mammalian Genetics, 1972; Elected Fellow of the Royal Society, 1985. Scientist Medical Research Council Mutagenesis Unit, Edinburgh,1959-62 and 1964-66; National Institute of Health Post-doctoral Fellow, Oak Ridge , Tennessee, 1962-64; Senior Scientist City of Hope Medical Center, Duarte, California, 1966-69; Senior Scientist Medical Research Council Radiobiology Unit, Harwell, Oxford, 1969-85; Head of Genetics Division, 1985-97; Director of the newly created Mammalian Genetics Unit 1997-2000. Author of over 250 published papers. Specialist subjects; chemical and radiation mutagenesis in mice, analysis of mutations, control of X-chromosome inactivation, sex determination, genomic imprinting.
It’s obvious that Dr. Cattanach knows what he’s talking about with genetics, which is why I was disappointed to come across the following sentence in an otherwise excellent article about inbreeding:
It seems fixed into the minds of human geneticists that every one of us harbours about 7 deleterious recessive genes. Hence the scientific basis for the taboos on inbreeding in humans. But the concept is now applied to dogs and claimed as a reason why inbreeding should be avoided. But were deleterious recessives to be so common in dogs, every one of us who breeds dogs would be finding abnormal pups in most litters, and this is patently not the case. Explanations offered for this have included the reductions in dog population numbers during World Wars, the small size of founding populations, and maybe even the tough association canines have had with man over historical times. All may have done much to weed out deleterious genes that were originally present!
I think that’s a loaded statement and an appeal to ignorance. It assumes that breeders have the ability to diagnose genetic disease in their litters and that such diseases act on puppies. It ignores the deleterious recessives that result in abortions at midgestation and stillbirths at term. Plus all those puppies that just “fail to thrive.” It also appears to confuse “common” with “ubiquitous;” since common is an imprecise term, I’ll point out that diseases that are billed as “epidemic” in humans are well below the rates of “rare” diseases in dogs. There are breeds where deleterious recessive disease is rampant and some times pervasive: cardiomyopathy in Dobermans, molera in the Chihuahua, hyperuricemia in the Dalmatian, etc.
While cancer is probably more complex than a single recessive allele, Mastocytomas (skin cancer) usually present in middle aged or older dogs. Mammary Tumors usually present with the highest frequency between 6-10 years of age and rarely prior to 2 years of age. Lymphomas usually appear in dogs over 5. Oropharyngeal Cancers set in between 7 and 11 years. Osteosarcomas present between 2 and 8 with 7 years being the average. In the show world, these dogs could be retired for a decade and have 4 generations of offspring before the breeder just might get wind of the disease.
Many diseases are also idiopathic or variable in expression. There’s also a strong bias to under report disease, even ones where there’s a DNA test for and no real bias against. How many diseased puppies do you just not hear about? I don’t think it’s stretch to say that there are far more diseases that have neither names nor DNA tests than there are named conditions or easily testable diseases.
If the question is “how many deleterious recessive diseases are there in dogs” and “how prevalent are they” then the answer will come with objective and comprehensive genome analysis and greater awareness of causal factors in disease. It will not come with a subjective “well we’d see more of it if it were really that common” statement that has really no grounding in either statistical relevance or observational fact.
We see in almost all species that the tolerance for inbreeding is actually quite low, with failures to sustain progeny after 30% COIs and up. I contend that if the danger zone between complete infertility and disease load is so low, so too is the observed line between apparent disease and fetus failure that most often goes undocumented. It should not sound radical to assert that the observed ratio of diseased dogs to healthy dogs completely neglects the dogs that fail before birth from being counted. Even if we had complete data, we’d still only be looking at the subset of deleterious recessive diseases that are not serious enough to cause fetus failure but serious enough to cause disease progression in a diagnosable fashion.
I also think the larger point Dr. Cattanach is seemingly trying to make, that dogs carry less of a disease load than humans, is false. Isn’t it the observation that dogs are both more inbred, suffer from higher levels of recessive disease, and have many analogous diseases to humans that makes them so attractive for scientists who are focused on curing human disease?
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