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Genetic Diversity Issues

Practical Genetics for Bull Terrier Breeders and Owners

Health Seminar presented to the Bull Terrier Club of America, October 10, 2002

Jerold S Bell, DVM, Tufts University School of Veterinary Medicine

Some breeders concerned with breed-wide genetic diversity propose only assortative mating and outbreeding to those least related. Assortative mating is breeding based on the phenotype, or appearance of the dog. Breeders should always use assortative mating by breeding like-to-like to solidify the traits that their dogs have, and breeding like-to-unlike to bring in traits that they desire.

Linebreeding is breeding dogs more closely related (a higher inbreeding coefficient) than the average of the breed, and outbreeding involves breeding dogs less related than the average of the breed. Linebreeding tends to increase homozygosity. Outbreeding tends to increase heterozygosity. Linebreeding can expose deleterious recessive genes through pairing-up, while outbreeding can hide these recessives. Outbreeding can prevent dogs affected with recessively inherited disorders due to heterozygosity. It does not eliminate the recessive gene, as the gene is propagated in carriers. Neither type of breeding alters the frequency of individual genes, just how they are distributed in the offspring. Selection, and not the types of matings used affect breed genetic diversity.

If two parents are both heterozygous (both Aa) for a gene pair, on the average, they would produce 25% AA, 50% Aa, and 25% aa. (These are averages when many litters are combined. In reality, any variety of pairing up can occur in a single litter.) If a prolific stud dog comes out of this litter, and he is homozygous aa, then the frequency of the “a” gene will increase in the population, and the frequency of the “A” gene will decrease. This is known as the popular sire syndrome. Of course, each dog has thousands of genes that vary in the breed, and everyone carries some deleterious recessive genes. The overuse of individual breeding dogs contributes the most to decreased diversity (population bottlenecks), and the increased spread of deleterious recessive genes (the founders effect).

Genetic diversity in a breed means breeder diversity. It is the varied opinion of breeders as to what constitutes the ideal dog, and their selection of breeding stock that maintains breed diversity. If some breeders favor a certain line of dogs, and other breeders favor different lines, then that maintains diversity. If someone breeds one line, and wants to outbreed and bring in genes from another line, that maintains diversity. Breeders should select the best from their breeding programs to maintain the quality of the breed. Selecting mediocre dogs, just because they are unrelated is not desirable. The first and foremost goal is always to produce quality dogs.

Attempting to continually outbreed to maintain diversity is a self-limiting practice. If everyone outbreeds, then eventually, there will not be any “unrelated line” to be found. Everyone will have a mixture of everyone else’s genes. The fallacy of using outbreeding to maintain genetic diversity is the belief that the diversity of a breed must be maintained in every single dog. Breed diversity requires breeders that maintain different linebreeding programs of quality dogs. Then a healthy breed-wide mix of linebreeding and outbreeding involving quality dogs, without an overabundance of single dog contributions, maintains diversity.

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