A very liberally edited version of an article by Jerold S. Bell, D.V.M. that appeared in the September 1992 American Kennel Club Gazette, "The Ins and Outs of Pedigree Analysis, Genetic Diversity, and Genetic Disease Control" followed by some personal observations.
Pedigree Analysis, Genetic Diversity
& Genetic Disease Control
Without exception all breeds of dogs are the result of inbreeding. Inbreeding has either occurred through natural selection among a small isolated population (i.e. the dingo) or through the influence of man breeding selected animals to derive specific traits. Either way intensive inbreeding is responsible for setting enough of the dominant traits that the resulting group breeds true to type. At which point a population of dogs can be said to be a breed.
Dogs actually have more genes than humans. Tens of thousands of genes interact to produce a single dog. All genes are inherited in pairs, one from the sire and one from the dam. If the inherited genes from both parents are identical they are said to be homozygous. If the pair of inherited genes are not similar they are said to be heterozygous. The gene pairs that make a German Shorthair breed true to type are obviously homozygous. However, variable gene pairs like those that control coat color, size, scenting ability, etc. are still heterozygous within the breed as a whole.
Line-breeding concentrates the genes of a specific ancestor or ancestors through their appearance multiple times in a pedigree. When a specific ancestor appears more than once behind at least one animal on both the sire's side and yet another animal on the dam's side homozygosity for that animal's traits are possible.
However, if this specific ancestor appears only through a particular offspring of the ancestor in question then the Breeder is actually breeding on this offspring of the ancestor rather than on the ancestor itself. This is why having many "uncovered crosses" to a specific ancestor (those that come through different offspring of this specific ancestor) gives the Breeder the greatest chance of making the desired traits of the specific ancestor homozygous.
Homozygosity greatly improves the chances that the resulting pups will in turn pass on the desired traits of the specific ancestor to their pups. When selecting pups from a line-bred litter the Breeder must choose pups that display the desired traits of the specific ancestor or they have accomplished little. In fact, if these traits are not present in a line-bred pup it is very likely that it inherited its genes from the remaining part of its pedigree and will be unable to breed true to type. Because the Breeder selected “out” for the pups that didn’t display this original ancestor’s traits.
Inbreeding significantly increases homozygosity, and therefore uniformity within a litter. One of the best methods of evaluating how successful a line-breeding has been is to gauge the similarity of the littermates as compared with pups of other litters with similar pedigrees. Considerable similarity among littermates tells the Breeder the genes have "nicked" or paired together as anticipated. The resulting pups will likely be able to pass these genes to the next generation.
Undesirable recessive genes are always masked by a dominant gene. Through inbreeding a rare recessive gene can be passed from a common ancestor on both the sire and the dame's side creating a homozygous recessive offspring. The resulting offspring actually displays the trait neither of their parents displayed (even though both of them carried it). Understand that inbreeding does not create undesirable genes it simply increases the chance that traits which are already present in a heterozygous state within the breed will be displayed.
Too many Breeders outcross as soon as an undesirable trait appears, blaming the problem on breeding "too close." Nothing could be further from the truth. In fact out-crossing insures that the undesirable trait will be carried generation after generation in a heterozygous recessive state only to rear its ugly head again and again. Therefore, the Breeder who turns away from breeding “close” is simply passing a known problem on to succeeding generations and future Breeders.
When an undesirable trait is "unmasked" the Breeder who does his breed a real service is the one that stays with his line long enough to rid it of the undesirable trait. By controlling which specimens within their line are used for breeding in succeeding generations this Breeder can eliminate the undesirable trait. Once the recessive gene is removed it can never again affect the Breeder's line. Inbreeding doesn't cause good genes to mutate into bad genes it merely increases the likelihood that they will be displayed.
The “inbreeding coefficient” (or Wrights coefficient) is an estimate of the percentage of all variable genes that are homozygous due to inheritance from common ancestors. It is also the average chance that any single gene pair is homozygous due to inheritance from a common ancestor. Our pedigrees display the inbreeding coefficient for each dog in the first 4 generations of a specific dog's ancestry. Each inbreeding coefficient is calculated from that dog's 10 generation pedigree.
Note: Inbreeding does not cause good genes to somehow mutate - it only increases the likelihood that existing genes will be displayed - allowing the Breeder the chance to eliminate what had previously been unseen in their particular line although it was always present.
We gauge the amount of homozygosity in an animal using their inbreeding coefficient (or Wrights coefficient) - which can be seen as an estimate of the percentage of all variable genes that could be inherited from common ancestors. It also gives us a mathematical value for the average chance that any single gene pair is homozygous due to inheritance from a common ancestor.
Our pedigrees display the inbreeding coefficient for each dog in the first 4 generations of a specific dog's ancestry. However, the inbreeding coefficients displayed for each dog in our pedigrees is in turn calculated from that particular dog's 10 generation pedigree. We can trace most of our dogs back more than 20 generations - some as far back as 35 generations.
Our Working Dogs
Four generation pedigrees that contain 28 unique ancestors for the 30 positions in the pedigree would obviously generate a low inbreeding coefficient. Yet a ten generation pedigree for the same dog might look quite different. If this dog were to have say 700 unique ancestors filling the 2048 positions in the pedigree the results for the same dog would be a much higher and truer inbreeding coefficient. Sometimes what appears to be an out-bred mix of genes in the first few generations (especially with owners naming their own dogs) ends up being a fine example of line-breeding when the pedigree is extended.
However, it must be remembered that simply knowing the inbreeding coefficient of a dog does nothing to help us understand which ancestors the dog is actually bred on. We know that the animal in question has many crosses to the same ancestors but we don't know which ancestors they are. To understand this, and to unlock the secrets of a dog's pedigree, we must do a homozygosity study.
A homozygosity study is not a percent blood calculation. The percent blood of a dog and its immediate ancestors is relatively easy to estimate but not that important. In fact, the dog will have 50% of its blood from it’s sire and 50% of its blood from its dam. But if these two dogs have no common ancestors the inbreeding coefficient would be 0%. Homozygosity is far more important in determining what traits a dog is capable of passing on to its offspring than percent blood but it is extremely difficult to calculate without the use of a computer.
So while knowing a dogs inbreeding coefficient is important in accessing its potential to throw its type we still need to clearly understand which dogs behind a particular dog are the most influential. Simply knowing how homozygous a particular animal is does nothing to help the conscientious Breeder understand this. To understand this and to unlock the secrets of a particular dog's pedigree we must do a homozygosity study. We need to know which ancestors the dog in question is bred on.
On more than one occasion we have seen pedigrees in which the most influential ancestor for a homozygous trait doesn't even appear in the first three generations. In this type of situation, it is not unusual for this particular ancestor to contribute 50% of the homozygous genes of the dog in question. In this case if a dog is 16% inbred one ancestor would be responsible for 8% or 50% of the dog’s homozygosity. It is of paramount importance for the dedicated Breeder to know not only the inbreeding coefficient for the resulting litter before the mating is done but also which dogs in the pup’s pedigree are influencing their genetic potential.
Far too many matings have been done only on the basis of physical appearance with little if any regard to the sire's and dam's respective pedigrees or the interplay between the two. Novice Breeders don't realize that individual dogs may share desirable traits but inherit them differently. This is especially true of polygenic traits, such as ear set, bite, or length of forearm. And many Breeders fail to understand that breeding dogs which are phenotypically similar but genotypically unrelated won't produce the desired traits in the current litter - and will actually reduce the chance of these traits being reproducible in the next generation.
Conversely, individual working dog with the same pedigree do not inherit exactly the same genes and will not breed identically. Dogs in a litter are no more similar than brothers and sisters in a human family. Think about it. If dogs have more genes than people and they are as dissimilar as human siblings need we worry so much about the “too close” we hear sounded by all those who know little or nothing about linebreeding. We regularly breed litters with a Wright’s Coefficient of more than 20% with superior results. There have been examples in working dogs of fine animals with inbreeding coefficients as high as 65%.
The secret is that all line-breedings must be made on a combination of performance, appearance and ancestry. If a Breeder is going to be successful in solidifying a certain trait they must rigorously select breeding specimens which display the desired trait and have similar pedigrees. In so doing Breeders have a chance of making this desired trait homozygous over time. This is the one key to successful line-breeding that is most often missed by unsuccessful Breeders.
In choosing a line of dogs within any bred it is wise to choose a line with "critical mass". Find a line within your breed where the most pre-potent individual was mated many times and produced many superior offspring. Without enough genetic diversity it will be more difficult to find animals within the line that do not also share the faults of the pre-potent individual. These are the faults the Breeder will have the most difficulty in eliminating.
No matter how limited the critical mass the Breeder must never breed animals that are poor examples of what the Breeder is trying to produce simply because they share common ancestors. Breeding “paper” is the quickest way to ruination and is largely responsible for the negative attitudes people have toward line-breeding. To a Breeder no dog is worth more than what it is able to produce. No amount of titles can overcome an animal’s inability to reproduce its own great traits. Look at the lack of production from Secretariat (a thoroughbred race horse).
Most beginning Breeders suspicion they should start with a brood bitch of a particular line, and they are correct. If at all possible the new Breeder should obtain females that come not just from the same important stud but actually come from the same Mother-line that is behind the stud in question. Instead of trying to get a bitch as close to the stud in question look for a pedigree in which the mothers of the sires are themselves from the same genepool. This is the female who will likely produce great pups.
In all mammals the females are "X" "X" and males are "X" "Y" which means that only females carry the genetic code particular to the part of the gene string that is missing in all males. Horse Breeders refer to it the "X Factor" and have demonstrated that the gene responsible for the large heart so many great racing stallions have can be traced back thru their mother-lines to a single mare that lived more than 100 years ago. If a stallion has an oversized heart - like Secretariat - this particular mare will show up in his mother-lines over and over again. The mares themselves don't have the large heart but they carry the gene for it on their “X” chromosome. Likewise, the stallions do not throw the large heart themselves.
And so it is with all Working Dogs. The bitches are far more important than the studs in carrying particular genes forward. Understand that this is true even if the genes most sought were originally found in a pre-potent male. The key for any successful Breeder is to isolate those females that carried his traits and breed off of them. It has been our experience that many important traits are indeed sex linked and carried by the dams from generation to generation.
Successful Breeders realize they are fighting "the drag of the breed," which is the tendency for all animals to breed back toward mediocrity. If it didn't work this way super species and super races would have developed long ago in every animal on earth. For instance, in human beings it is impossible to breed parents with high IQs together to produce higher IQs. Even when two genius have children the average IQ of their children will be half way between normal and the average of their IQs.
By the way, Einstein, himself, was the off spring of parents who were themselves first cousins . . . and he married his first cousin. So much for the tails of woe you heard in school about the effects of inbreeding. In fact, the history of working dogs is replete with many examples of intensive inbreeding that produced some of the more influential dogs in our breed.
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