07/03/2016
Solving the problem of genetic disorders in dogs
6/2/2015
By Carol Beuchat PhD
If you breed purebred dogs, and if you care both about the health of the puppies you produce as well as the future of the breed, you need to understand why the list of genetic disorders in dogs continues to get longer and longer.
It's not because of environmental toxins.
It's not because of vaccines.
It's not because of commercial dog foods.
It's not because of backyard breeders and puppy mills.
It's not because of better diagnostics and veterinary care.
What's the problem?
Among biologists, veterinarians, and genetic researchers, there is no controversy over the cause of the growing list of genetic disorders in dogs. The genome of every animal harbors many mutations that are passed from generation to generation with no harmful effects because they are recessive. Most genetic disorders in dogs are not caused by a new mutation, but by mutations that are ancient, originating in some animal hundreds or even thousands of generations ago, or perhaps even a legacy of the progenitor wolf. These mutations result in a genetic disorder when a dog inherits two copies. The increasing burden of genetic disease in purebred dogs is a direct and predictable consequence of breeding practices that increase the expression of deleterious alleles.
Here are some examples of what the experts say:
"Intense selection, high levels of inbreeding, the extensive use of a limited number of sires, and genetic isolation are all hallmarks of modern breeds of domestic dog. It is widely agreed that part of the collateral damage from these practices is that purebred dogs have a greater risk of suffering from genetically simple inherited disorders than their cross-breed cousins." (Mellersh 2012)
"The indirect effects of selective breeding for appearance include very significantly reduced genetic diversity unevenly spread across the genome, resulting in elevated prevalence of specific diseases within particular breeds. Coupled with ill-advised breeding practices and insufficient selection pressure on health and welfare, this has led to certain breeds becoming especially susceptible to a whole suite of disorders, many of which are acutely painful or chronically debilitating...Since many diseases are the consequence of homozygosity for recessive alleles, breeding of close relatives is accompanied by a corresponding increase in the occurrence of these disorders...because an animal must inherit one defective gene from each parent in order to develop the condition. When parents are closely related, the liklihood of them both carrying a copy of the same deleterious gene is significantly elevated." (Rooney & Sargan 2009)
"Since their domestication, dogs have undergone continual artificial selection at varying levels of intensity, leading to the development of isolated populations or breeds. Many breeds were developed during Victorian times and have been in existence for only a few hundred years, a drop in the evolutionary
bucket. Most breeds are descended from small numbers of founders and feature so-called popular sires (dogs that have performed well at dog shows and therefore sire a large number of litters). Thus, the genetic character of such founders is overrepresented in the population. These facts, coupled with breeding programs that exert strong selection for particular physical traits, mean that recessive diseases are common in purebred dogs and many breeds are at increased risk for specific disorders." (Ostrander 2012)
"Conditions not relating directly to breed standards account for over 75% of all inherited disorders in pedigree dogs and have been attributed to breed formation and small effective population size, the repeated use of popular sires and inbreeding. The development of the breeds has been associated with the increasing prevalence of a large number of genetic diseases." (Farrell et al 2015)
Why aren't we solving the problem?
Adding to the problem of management of genetic disorders in dogs is the fact that in many cases breeders don't know what to do about them. The research and veterinary communities realize this:
"Breeding against these inherited conditions becomes therefore a major concern for owners and breeders. However, despite an increasing number of gene tests available, breed clubs often do not know which strategy to adopt, especially concerning the use of interesting studs that may be disease carriers, and are unaware of the impact of such policies on genetic diversity." (Leroy and Abitbol 2010)
"Doing a genetic test and subsequently eliminating an individual from the breeding population may not be the best strategy, as by targeting a particular allele at one genetic locus for removal from the gene pool of a particular breed, breeders may in fact increase allele frequency of genetic variants on alternative haplotypes at the same, or a different locus, that are recessively deleterious. In addition, by eliminating some animals from breeding, a reduction in the effective population size will occur, thus risking higher levels of inbreeding, potential founder effects and genetic bottlenecks. In essence, by correcting one problem there is a chance of inadvertently creating a new one. [A] three-pronged strategy, incorporating new and current screening schemes, pedigree information, and EBVs or gEBVs, could reduce the number and prevalence of inherited disorders, while at the same time genetic diversity can be managed. This is particularly important in rare breeds with a small or decreasing population size and for breeds predisposed to a high number of inherited disorders." (Farrell et al 2015)
"Genetic disease in purebred dogs - what a fine state of affairs!...How did it happen? Why is it like this? The answer to both questions is easy, and it is the same answer in both cases. It happened and is like this because, over the years, almost no one has tried to do anything to control genetic disease in dogs. As a matter of fact, most of the things that they did inadvertently, not (I hope) intentionally, tended to foster genetic disease." (Padgett 1998)
"As the genetics underlying complex disorders are revealed, canine breeders and their registering organizations will be required to understand genetics in a much more sophisticated way. To facilitate the management of genetic disorders in the era of new complex information, we [need to] consider how best to apply the results of new research and analytical techniques to benefit the wider canine breeding community...If this is not done, there is a serious risk that expensive and valuable genetic research will remain unused or be misused to the detriment of breeds...Even when selection is undertaken for health- and welfare-related goals (i.e., animals free of known disorders), care must be taken to ensure that the reduction in effective population size through the use of popular breeding animals does not cancel out the health and welfare gained by selection against disorders. A major challenge in improving welfare for all dogs is to assist breeders in making balanced and informed breeding choices relating not only to recessive Mendelian disorders, but also to multifactorial genetic disorders...Thus far, the canine breeding community has been tantalized by the opportunities presented by new technologies to tackle existing disorders, but has been given scant advice on how to incorporate information arising out of research into their breeding program." (Wilson and Wade 2012)
What should we do?
Clearly, if the best, most diligent efforts of responsible breeders are not solving the problem, then doing more of the same isn't going to improve things. Among the "solutions" that will NOT solve the problem are:
stricter selection of which dogs are allowed to bred
reliance on knowing what's in your lines
more studies of the diseases, especially ones like cancer and epilepsy
elimination of the deleterious alleles from the gene pool
research to identify the "bad" genes
development of more genetic tests
ignore the problems
hope
You probably see some of your strategies on this list. But none of these address the roots of the problem as indicated by the scientists and veterinary professionals above, and as they point out, many of these things actually make the problem worse. They also point out, though, that while breeders are being offered new tools like DNA tests, they are given insufficient instruction in how to use them properly.
Even worse, many times breeders don't have the basic information and expertise they need to understand the current genetic status of their own breed. For example, "care must be taken [not to reduce] effective population size". Do you know the effective population size of your breed? (Do you know what "effective population size" means?) Are you using EBVs to improve the efficiency of selection against genetically complex diseases? (Do you know what "EBV" means?) Do you know the distribution of founder alleles in your current breeding population? (Do you know your breed's founders?) Do you know that most genetic disorders in dogs are caused by recessive mutations and that they are completely avoidable? (Do you understand why?)
No amount of money - and by now it must be millions per year - invested in more research and more testing is going to stem the tide of genetic disorders in purebred dogs. That seems clear. So how are we going to solve this problem?
The solution will come - and can only come - from the breeders. Not by doing more of the same, but by recognizing that traditional breeding practices have created the problem, and that breeders will need to learn about breeding strategies that will allow them to produce puppies that meet their goals as a breeder and will also live long, healthy lives.
Picture
From Rooney & Sargan 2011. Pedigree dog breeding in the UK.
The solution is EDUCATION
"As the genetics underlying complex disorders are revealed, canine breeders and their registering organizations will be required to understand genetics in a much more sophisticated way."
If you breed dogs, and if you consider yourself a responsible breeder, you will need to learn more about genetics. You will need to understand not just classical Mendelian genetics, but also population genetics and some basic molecular genetics. Appropriate courses should be available at any university, or you might find them at your local community college. As far as I am aware, ICB is the first (and only) to offer courses in population genetics expressly designed for dog breeders, but you might find comparable course material in courses for hobby breeders of horses, cats, alpacas, or other domestic animals. If you have some background already in biology, you can learn from many books that aren't specifically about dog breeding but cover the relevant material. (I can recommend some that are appropriate for your particular background).
It won't be enough for you to learn all of this new stuff while your fellow breeders do nothing. You are all family through your shared gene pool, and improving the health of dogs will necessarily be a cooperative, community effort. A single hugely popular sire can devastate the gene pool of a breed in a generation, and breeders need to recognize that they will all share in the genetic tragedy that is likely to become apparent a few generations down the line. Find some fellow breeders that share your concern for the future of your breed and sign up for a class together. Join ICB's Breeding for the Future group, or a comparable forum moderated by scientists, not a bunch of fellow breeders who have lots of opinions but little knowledge. Pay attention to the research progress in canine genetics, so you will be aware of new developments and opportunities for you to further your knowledge.
Of the many stake-holders in the health of purebred dogs, the ones that will - and must - ultimately solve the problem are the breeders. For this you will need education. There's a lot for you to learn, so make a pledge to start now.
Farrell LL, JJ Schoenbeck, P Wiener, DN Clements, and KM Summers. The challenges of pedigree dog health: approaches to combating inherited disease. Canine Genetics and Epidemiology 2:3.
Leroy G and M Abitbol. 2010. Breeding strategies against genetic disorders in dog breeds.
Mellersh C. 2012. DNA testing and domestic dogs. Mammalian Genome 23:109-123.
Ostrander EA. 2012. Both ends of the leash - the human links to good dogs with bad genes. New England Journal of Medicine 367: 636-646.
Padgett GA. 1998. Control of canine genetic diseases. Howell Book House, NY.
Rooney, N and D Sargan. 2009. Pedigree dog breeding in the UK: a major welfare concern? A report commissioned by the the Royal Society for the Prevention of Cruelty to Animals.
You can learn more about the genetics of dogs in ICB's online courses.
*** "Managing Genetics for the Future" starts 8 June ***
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