By Carol Beuchat PhD
happened to run into this post of mine in a from years ago that I had long since forgotten about. It was originally posted to a discussion about health problems in some breed (I don't remember which) that breeders were struggling with without success. Note that I just blogged about this same topic a few days ago.
. . . . . . . . . . . . . . . . . . . . .
Here's the problem.
Breeders are assessing the issues they see in a breed and taking actions to address them based on what they know from their experience as breeders.
If you develop a cough, you pull out the remedy you usually use and wait for it to go away. But if that cough is tuberculosis and not a simple cold, your remedy will not solve the problem and the consequences could be serious. You need to see a doctor, and you need to take the proper medication.
. . . . . . . . . . . . . . . . . . . . .
Here's the problem.
Breeders are assessing the issues they see in a breed and taking actions to address them based on what they know from their experience as breeders.
If you develop a cough, you pull out the remedy you usually use and wait for it to go away. But if that cough is tuberculosis and not a simple cold, your remedy will not solve the problem and the consequences could be serious. You need to see a doctor, and you need to take the proper medication.
| Breeders are aware of the issues in their breed. They respond to these using the tools they are familiar with from breeding, which generally involve culling dogs affected with a genetic problem and breeding away from dogs suspected to be carriers of the genes thought to be causing the health problem. This is treating the cough as if it is a simple cold. In fact, the mutations causing the genetic disorders are not the problem. If we focus on the mutations instead of addressing the actual problem, you will never win. Every dog has mutations, some you know about and many that you won't until they become a problem. Trying to improve health by targeting these one by one is a game of genetic whack-a-mole you will never win. We are not winning because we are not focused on the source of the problems. |  |
So, what is the source of the problem? Animals in closed populations can only breed to their relatives. All breeding to related dogs is inbreeding. Inbreeding produces homozygosity - two copies of the same allele at a locus. This is a good thing for the genes for type. It's a bad thing for genes that are broken. A genetic disorder is not caused by mutation bombs that you can simply remove to restore health. A genetic disorder is what results when a dog does not inherit a copy of the allele necessary for proper function. So, we have some loci that have two copies of a good allele; but for all the loci that have two copies of a broken allele, something function will be broken.
We are trying to produce healthy dogs by throwing mutations out of the gene pool. But it's a closed, finite gene pool; eventually we will throw all of the genes out. In fact, animals in closed populations go extinct.
Again. Animals in closed populations - aka closed gene pools - go extinct.
There is no "breeder magic" that will prevent this. There is no "science magic" that will prevent this. Animals in closed gene pools go extinct. Some sooner, and some later, but inbreeding will relentlessly increase over time, and diversity will decline, until so much stuff is broken that the animals can no longer reproduce and survive.
All of the other things breeders usually discuss really aren't relevant to addressing this overarching, unavoidable problem. Should we worry about hip scores, or is longevity more important? What about an eye problem that has a late onset? What about mutations with only mild effects? There is lots to talk about, and discussions have continued...for years.
But here's the only problem we need to talk about: inbreeding and loss of genetic diversity.
Fix this problem and you will have healthy dogs. If you start with a population of healthy dogs and randomly remove 40% of the alleles the breed started with, you will most certainly break things. This breed's average inbreeding is more than 40%; if half of that (20%) is homozygosity for good genes, then 20% of it is homozygosity for bad genes. That's a lot of stuff that's broken.
You cannot select your way out of this problem; remember, selection removes alleles, and lost alleles are the problem. It might be possible to restore some lost genetic diversity by strategically using less closely related dogs for breeding. Genetic analyses can reveal if this is possible.
We have much better tools to guide breeding decisions now than simply looking at stacks of pedigrees and comparing health issues. At the very least, you should be using those. You should know the heritability of all of the traits and disorders under selection (0.06? 0.33? 0.89? You should know the size of your gene pool (is it 57, 18, or 6?). You should know the effective population size of the breed (504? 92? 4?). You should know the pairwise kinship of the breeding dogs in the population; the inbreeding data suggest that the dogs are on average as closely related as what you would get from 3 or 4 consecutive full-sib crosses. Would you ever do 3 or 4 full-sib crosses???? In terms of genetics, that's what you have. You need to know which dogs in the breeding population have the highest genetic value so you can be sure to breed those, and which have the lowest value so they can be retired. You should know how much improvement in all of these things is possible if the existing genetic diversity in the breed is used in the most strategic way. If it turns out that this will not be adequate to restore the breed to health, then you need to evaluate strategies that will.
These are things you won't learn about in 20 or 30 years of breeding. You probably don't know anything about effective population size or kinship coefficients or founder genome equivalents. These are not things you will learn by breeding. These come from the science of population genetics that has been developed over the last 100 years by study of thousands and thousands of breeding programs for both domestic and wild animals. These are the tools used by breeders of other domestic animals. They are used in genetically managed programs for service dog breeding; they work for dogs just as they do for any other animal.
To solve the problems in this breed and in purebred dogs, we will have to correctly identify the cause of the problem (inbreeding and loss of genetic diversity), determine the best strategies for addressing the problem, and design a breeding strategy to effectively and efficiently restore the breed to health.
We have the tools and expertise to do this. We could be doing this NOW.
We are trying to produce healthy dogs by throwing mutations out of the gene pool. But it's a closed, finite gene pool; eventually we will throw all of the genes out. In fact, animals in closed populations go extinct.
Again. Animals in closed populations - aka closed gene pools - go extinct.
There is no "breeder magic" that will prevent this. There is no "science magic" that will prevent this. Animals in closed gene pools go extinct. Some sooner, and some later, but inbreeding will relentlessly increase over time, and diversity will decline, until so much stuff is broken that the animals can no longer reproduce and survive.
All of the other things breeders usually discuss really aren't relevant to addressing this overarching, unavoidable problem. Should we worry about hip scores, or is longevity more important? What about an eye problem that has a late onset? What about mutations with only mild effects? There is lots to talk about, and discussions have continued...for years.
But here's the only problem we need to talk about: inbreeding and loss of genetic diversity.
Fix this problem and you will have healthy dogs. If you start with a population of healthy dogs and randomly remove 40% of the alleles the breed started with, you will most certainly break things. This breed's average inbreeding is more than 40%; if half of that (20%) is homozygosity for good genes, then 20% of it is homozygosity for bad genes. That's a lot of stuff that's broken.
You cannot select your way out of this problem; remember, selection removes alleles, and lost alleles are the problem. It might be possible to restore some lost genetic diversity by strategically using less closely related dogs for breeding. Genetic analyses can reveal if this is possible.
We have much better tools to guide breeding decisions now than simply looking at stacks of pedigrees and comparing health issues. At the very least, you should be using those. You should know the heritability of all of the traits and disorders under selection (0.06? 0.33? 0.89? You should know the size of your gene pool (is it 57, 18, or 6?). You should know the effective population size of the breed (504? 92? 4?). You should know the pairwise kinship of the breeding dogs in the population; the inbreeding data suggest that the dogs are on average as closely related as what you would get from 3 or 4 consecutive full-sib crosses. Would you ever do 3 or 4 full-sib crosses???? In terms of genetics, that's what you have. You need to know which dogs in the breeding population have the highest genetic value so you can be sure to breed those, and which have the lowest value so they can be retired. You should know how much improvement in all of these things is possible if the existing genetic diversity in the breed is used in the most strategic way. If it turns out that this will not be adequate to restore the breed to health, then you need to evaluate strategies that will.
These are things you won't learn about in 20 or 30 years of breeding. You probably don't know anything about effective population size or kinship coefficients or founder genome equivalents. These are not things you will learn by breeding. These come from the science of population genetics that has been developed over the last 100 years by study of thousands and thousands of breeding programs for both domestic and wild animals. These are the tools used by breeders of other domestic animals. They are used in genetically managed programs for service dog breeding; they work for dogs just as they do for any other animal.
To solve the problems in this breed and in purebred dogs, we will have to correctly identify the cause of the problem (inbreeding and loss of genetic diversity), determine the best strategies for addressing the problem, and design a breeding strategy to effectively and efficiently restore the breed to health.
We have the tools and expertise to do this. We could be doing this NOW.
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ICB's online courses
***************************************
Visit our Facebook Groups
ICB Institute of Canine Biology
...the latest canine news and research
ICB Breeding for the Future
...the science of animal breeding
