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Why Incest isn't Best
the breeding of purebred dogs is akin to [breeding laboratory mice]…[most breeds] are becoming progressively more inbred. My observation is that most are on the road to extinction, but most breeders do not even realize they are part of an experiment. John B. Armstorng, PhD
Incest, the marriage of close relatives, is forbidden on moral and legal grounds in most human societies. But the prohibitions also make biological sense. Inbred individuals tend to suffer from inbreeding depression. Among humans, the ancient Egyptian pharaohs and the more recent Hawaiian and European royal families provide the better known human examples. Inbreeding depression has not been studied much in dogs but it is well researched in other species.
Many dog breeders know very little about inbreeding depression. Purebred dogs, including the Australian Shepherd, are commonly bred to related individuals and occasionally bred to relatives close enough that among humans such a match would be considered incest. These are dogs, not people, so the moral issues are not the same. However, we ignore the biological impacts of the practice at our peril—or more accurately the peril for our dogs.
Inbreeding depression is the loss of viability or function resulting from excess inbreeding. The signs of inbreeding depression most frequently cited are reproductive failures. Anything that impacts reproduction is an up-front concern for dog breeders. But inbreeding depression may also manifest itself as poor health. Its effects are often so subtle that breeders or even veterinarians may not recognize the root cause and not every inbred animal will show signs of it.
Inbreeding-related reproductive failures include lack of libido and low sperm count in males while females may fail to get pregnant, have unusually small litters, or exhibit poor mothering ability. Non-reproductive indications of inbreeding depression can be manifested as a high frequency of immune-mediated diseases, significantly higher incidence within a line or breed of one or more diseases than is seen in the species as a whole, or even things as subtle as dogs that is are “poor-keepers” or that seem to catch every little bug that comes along. In really severe cases, a very inbreed strain may go extinct.
The level of inbreeding is usually measured using a formula called Wright’s Coefficient of Inbreeding. It calculates the probability that genes may have been inherited from both sides of an individual’s pedigree. It is far too complex to do by hand over more than two or three generations, but some of the better pedigree software will calculate coefficient of inbreeding (COI) for you. The usual 3-5 generation pedigree won't give sufficient information for a useful calculation. For Australian Shepherds the author has found 10 generations to be the best indicator. Few Aussies have a complete 10-generation pedigree, so this will give you pretty much all there is to know. While some individual lines of descent may go back 20-30 generations, running the calculation for more than ten does not result in any significant change in the result. Running it for fewer than 10 frequently causes the number to drop, giving an incorrectly optimistic result.
Modern breeders should know the COI of each of their dogs and determine what the COI will be on planned litters. The average COI is for the breed, or at least the portion of it with which you work, is also important. Show line Aussies average around 12-14%. This is roughly equivalent to all of them being half-brothers and sisters. Working line Aussies are variable. Many have very low COIs, under 6%, but some are as high or higher than the show line average. Inbred dogs, especially those with a COI of 25% or higher, that are experiencing problems of the kind described above may be exhibiting inbreeding depression.
Not surprisingly, most the research on the effects of inbreeding in mammals has been done on farm animals, laboratory animals like mice, and endangered wild species.One study on cattle done by Dr. John Pollock at Cornell University indicated increasing levels of health and productivity problems as the COI rose beyond 9%.
Any time a single dog or kennel is responsible for producing a significant portion of the breeding animals in a breed, the breed gene pool is skewed toward whatever genes those dogs had. Breeders do this because they want to skew things toward genes that produce the desirable traits they found in those animals. But along with those genes come others that are less desirable.
Inbreeding depression isn’t a matter of specific genes that cause the sort of health problems we are always on the lookout for, like bad hips or eye disease. It lies in genes that have a subtler effect. Such genes work only a little less well than their “good” versions.A breeder isn’t likely to notice a 5-10% reduction in bodily function. But as levels of inbreeding rise these genes may accumulate. Over time reproductive and health problems increase. The changes are so gradual they are often blamed on diet, pollution and other environmental causes. All of these may contribute but it is genes that make dogs susceptible.
Every individual has 3-5 “lethal equivalents.” These are an accumulation of mutated genes that, if matched with like genes, would either kill the organism early in life or prevent it from reproducing. The portion of lethality carried by any one gene can range from 1-100%. A single lethal equivalent may be one gene, 10 that reduce fitness by 10%, or a hundred that reduce it 1%. The mutt that lives down at the corner probably has in excess of 100 such genes. Your purebred Aussie likely has more because they have been inadvertently concentrated by inbreeding.
Each puppy will inherit half of its lethal load from each parent. If the parents are unrelated, there may be little or no impact on that puppy. But if they are, the risk of pairing up genes that are part of the load increases. The more ancestors there are common to both sides of the pedigree, the greater the risk. This is why monitoring COI is important.
Historic inbreeding and recent or “close” inbreeding (matings that would be considered incestuous among humans) are the primary factors contributing to inbreeding depression in purebred dogs, with the historic inbreeding being the greater factor since many don’t recognize it as inbreeding at all.
There is a strong possibility that inbreeding depression may be becoming a factor in Australian Shepherds. Reproductive problems do occur. Almost 25% of the breeders who responded to the 1999 Australian Shepherd Club of America breed health survey reported having dogs with reproductive failures (low sperm count, lack of libido, failure to conceive, small litters or poor mothering ability). This has sobering implications for the breed. There is no way from this data to know the COIs of the affected dogs, but such a large response indicates that breeders need to pay more attention to reproductive issues.
Autoimmune disease data was gathered in the same survey. (Allergy data is being excluded here because the survey did not distinguish between mild or isolated allergy attacks and severe chronic conditions.) 17% of the respondents reported having had at least one dog with autoimmune disease, another sobering statistic.
Understanding the problem is one thing, but what can a breeder do about it? Very few practice close inbreeding generation after generation. And breeders aren’t in the business of breeding COIs, they want to produce quality dogs. This can be accomplished by giving preference to assortative mating—the breeding of individuals of the desired phenotype with preference given to those least related.
Say your bitch as a COI of 12%. You have looked at possible studs and narrowed your list to three that you think are equally good. Dog A is from the same line as your bitch; the puppies would have a COI of 21%. Dog B, an older stud who hasn’t been bred often but has produced consistently good offspring, would produce puppies that are 10%. Dog C, an up-and-coming star already booked for a number of bitches, would produce puppies that are 8%. Dog C might seem like the way to go at first glance, but he has high potential for becoming a popular sire. Your puppies might wind up half-siblings to everybody else’s. Dog A causes a significant increase in inbreeding. Dog B may not give the lowest COI, but he does produce puppies that will be lower than their dam and you are more likely to find less-related mates for them because he has not been over-used.
We also need to do what we can to reduce the lethal load carried by our dogs. We should not use animals that exhibit signs of inbreeding depression. Consistent reproductive failures are an excellent argument against further breeding. There is nothing more natural than reproduction. The dog that is unable to produce, deliver or rear puppies without veterinary intervention or considerable help from the breeder should not be bred. Chronically unhealthy or unthrifty dogs are not good breeding candidates, even if they have no identifiable hereditary disease. Nor are dogs with severe allergies or any chronic autoimmune disease. Some of these diseases can affect reproduction, thyroid disease being a prime example. In the author’s opinion, the administration of thyroid hormone to bitches that will not settle so they can produce puppies is foolishly short-sighted at best and highly unethical if done by someone knowledgeable.