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
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,