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.
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 dame. 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.
Linebreeding 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 dame'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 breed 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 linebred 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 linebreeding 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.