In parts one
of this series, we presented an overview of the basic building blocks of genetics and then looked at methods of pedigree analysis that can be especially useful in researching potential breedings.
This study has taken us from the level of the individual dog – that is, understanding how the chromosomes in every cell of the dog's body encode for his unique genetic blueprint – to that of his extended canine family. With the help of pedigree research and consideration of the traits of ancestors and relatives, we gain some insight regarding traits that a dog may, in turn, pass along to his offspring.
In this third installment, our final step is to consider even broader dynamics of breeding decisions – that is, looking at potential matings in relation to the breed's overall population. Line-breeding or Inbreeding?
Although the terms inbreeding and line-breeding are used frequently in discussions among serious dog fanciers, the distinction between the two can be rather hazy. In genetic terminology, inbreeding refers to the mating of two animals who are related to each other. Its opposite is outcrossing (or outbreeding) – the breeding of animals who are wholly unrelated.
It's worthwhile to keep in mind, however, that all dogs within a breed are ultimately related. Dr. Jerold Bell, director of the Clinical Veterinary Genetics course at the Tufts University School of Veterinary Medicine, explains:
"While a mating within a breed may be considered outbred, it still must be viewed as part of the whole genetic picture: a mating within an isolated, closely related, interbred population. Each breed was developed by close breeding and inbreeding among a small group of founding canine ancestors, either through a long period of genetic selection or by intensely inbreeding a smaller number of generations. The process established the breed's characteristics and made the dogs in it breed true."
To geneticists, any mating of related animals is considered inbreeding. In the breeding of purebred livestock, however, the word inbreeding generally refers to matings of very closely related animals, such as parent-offspring or full siblings, and line-breeding describes a milder form of inbreeding, where the breeding pair is related but more distantly so, such as the breeding of cousins or of grandsire to granddaughter. What is the genetic effect of line-breeding or inbreeding?
Basically, it increases the probability that the two genes in any given gene pair in a dog's genetic makeup (that is, the two alleles at a particular locus) will be identical and derived from the same ancestor. As we learned in Part One, a dog's "genetic blueprint" consists of many gene pairs, each pair consisting of one gene inherited from the sire, and one from the dam. When the two genes of a pair are identical, the dog is said to be homozygous for that particular trait; when there are differences between the two genes of a pair, the dog is heterozygous for that trait.
Being homozygous for an individual trait is neither inherently good nor inherently bad in itself. It is important to remember that inbreeding does not "create" bad or undesirable genes.
When a close breeding produces a high degree of homozygosity in offspring, this can coincide with a high degree of uniformity in the litter and can result in animals who will strongly "breed true."
Inbreeding can, however, increase the chance of expression of undesirable traits that are "hidden" or passed along harmlessly when part of heterozygous pairs. Included in this is the possibility of exacerbating the expression of inherited disease. Coefficients of Inbreeding (COI)
It can be very useful to know just how inbred a potential breeding animal is – in other words, to know the degree of probability that the dog is homozygous for certain traits, whether desirable or undesirable. Getting a sense of this simply by going over the dog's pedigree can sometimes be surprisingly difficult, because common ancestors may be hidden many generations back.
Fortunately today's breeders have tools available to help in this. American geneticist Sewall Wright (1889-1988), a founder of population-genetics theory, described a value called the coefficient of inbreeding and developed methods of calculating it in pedigrees.
The coefficient of inbreeding (COI), expressed in a value ranging between 0 and 100 percent (or 1), represents the probability that both genes within variable gene pairs are derived from the same ancestor, with 0 representing a true outcross (that is, the offspring of two truly unrelated animals) and 100 percent representing an extremely inbred animal, such as would result from many repeated generations of brother-sister matings (very rare in mammals, and seen only in inbred strains of laboratory rodents). The COI is also the average chance that any given gene pair is homozygous due to inheritance from a common ancestor.
Until recent years, computing a dog's COI over multiple generations entailed going through an enormous stack of pedigrees and doing extensive calculations by hand, so the equation wasn't used much by dog breeders. Now, however, a number of pedigree-software programs, including The Breeder's Standard (compuped.com) and Pedigree Explorer (breedmate.com) can compute pedigrees of 20, 40, and more generations and determine the COI reflecting your dog's degree of inbreeding to particular ancestors.
In considering breeding decisions, it's important to realize that the COI represents degree of relation to a specific ancestor or ancestors. If your bitch has a high COI to a specific ancestor, you might not wish to breed her to a male who goes back to that same dog. However, breeding her to a male from very different lines – even if he himself has a high COI, but to a different ancestor – can result in offspring with very low COIs. Using the Information: Making Breeding Decisions
Dr. Bell offers important advice on breeding strategies relating to line-breeding/inbreeding and COI:
"Decisions to inbreed, line-breed, or outbreed should be made based on the knowledge of an individual dog's traits and those of its ancestors. Inbreeding will quickly identify [both] the good and bad recessive genes the parents share in the offspring. Unless you have prior knowledge of what the pups of milder line-breedings on the common ancestors were like, you may be exposing your puppies (and puppy buyers) to extraordinary risk of genetic defects. In your matings, the inbreeding coefficient should only increase because you are specifically line-breeding (increasing the percentage of blood) to selected ancestors.
"Don't set too many goals in each generation, or your selective pressure for each goal will necessarily become weaker. Genetically complex or dominant traits should be addressed early in a long-range breeding plan, as they may take several generations to fix. Traits with major dominant genes become fixed more slowly. Desirable recessive traits can be fixed in one generation, because individuals that show such characteristics are homozygous for the recessive genes and you will be able to see them. Dogs that breed true for numerous matings and generations should be preferentially selected for breeding stock. This makes them prepotent and of extreme value." Keeping It All in Perspective
Buoyed by their passion for the breed, serious breeders devote themselves to ongoing study and careful consideration of breeding decisions. Along with all the work that "doing it right" entails is the reward of ensuring the breed's preservation into the future. Pembroke Welsh Corgi breeder Ann Bowes, 2007 Herding Group Breeder of the Year, shared the following thoughtful insights in the Spring 2008 issue of AKC Breeder:
"Linebreeding, inbreeding, and outcrossing are tools the experienced breeder uses to set type. A thorough knowledge of the pros and cons of these methods and when to use them are necessary for an ongoing and productive breeding program…Generating pedigrees that keep producing dogs with type and quality takes years and years of hard work and self-discipline. However, I cannot conceive of any endeavor that gives more pleasure and fulfillment than creating beautiful, healthy, and affectionate dogs who are a real credit to the breeding programs that produce them."
Arliss Paddock breeds and shows English Cocker Spaniels and is former managing editor of the AKC Gazette.