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Maintaining and Improving Breeds

Jerold S Bell DVM, Cummings School of Veterinary Medicine at Tufts University jerold.bell@tufts.edu
In order to understand how to maintain breeds, we have to mentor their puppy buyers to expand their breeder
to understand the genetic forces that shape them. Natu- base as well as the number of dogs.
ral species evolve through natural selection. Any genetic
changes within a population that improve the chance of Population expansion allows the creation of new “family
survival and ability to reproduce in the populated envi- lines.” A larger population allows the average relatedness
ronment will be at an advantage and thrive. This results in of breeding pairs (based on recent generations) to be less
a loss of genetic diversity through the disadvantaged. This than the prior generation. Population contraction is det-
loss is not detrimental to the population as it is directly rimental to breed maintenance due to the loss of quality
related to increasing its superiority. breeding lines and genetic diversity. Healthy breed gene
pools require expanding, or large, stable populations.
Dog breeds develop through
artificial selection for desired There are times when a lot
phenotypes – what you can of breeding is going on and
see in the dogs. These can in- registrations are increasing,
clude conformation, behavior, and times (such as the recent
working ability and health. past) when less breeding is
Most breeds originally started going on. However, it is the
from either a small population offspring that reproduce
of related founders, or as a (regardless if from prolific
population of unrelated dogs or limited-breeding parents)
that conformed to a working that contribute their genes to
or conformational pheno- the next generation. Breed-
type. Some breed lines will ing quality dogs from differ-
be discarded over time due ent “lines” and areas of the
to genetic defects, or an inability to adhere to a standard. gene pool prevents the loss of genetic diversity.
Regardless of the breed origin, generations of reproduc-
tion within a small population produce homozygosity The popular sire syndrome is the single most influential
(the fixation of gene pairs) through close breeding. This factor in restricting breed gene pool diversity. When a
is what causes breeds to reproduce themselves with each breed is concentrating on a specific sire or multi-gener-
generation. ational sire line, other quality male lines are abandoned.
This causes a loss of genetic diversity to the breed gene
Genetic studies of dog breeds show that they lose on aver- pool in exchange for a rapidly increasing influence of the
age 35% of their genetic diversity through breed forma- popular sire.
tion. Genetic studies also document the increased homo-
zygosity found in dog breeds. Low effective population Now is an important time to use frozen semen of qual-
size (low number of founders) and high deep-pedigree ity dogs from the past to expand gene pools. Stored DNA
inbreeding coefficients (homozygosity) are a natural (such as from the OFA CHIC repository) or semen can be
and expected consequence of breed development. used for breed-specific genetic testing that might not have
been previously available.
Breeds differ from natural populations in that only a small
percentage of dogs reproduce to create the next genera- All individuals carry some deleterious genes, which can
tion. In a population sense, this represents a genetic increase in frequency with natural as well as artificial
bottleneck with each generation. Individuals chosen for selection. More “lines” of naturally occurring species have
breeding should represent the quality traits of the breed. died off due to genetic disorders or diminished fitness
Quality traits should not be lost through the absence of than those that have survived. As individuals propagate,
selection or the abandonment of quality lines. deleterious mutations can become breed-related disease if
they are disseminated and increase in frequency.
Population expansion is an important aspect of breed
maintenance. If the offspring of small population breeds Studies show that some breeds have more issues of spe-
are generally healthy their population can grow and cific genetic diseases with linebreeding and others do not.
expand. They are at stages of breed development where This depends on the genetic load of deleterious recessive
more populous breeds were earlier in their devel- genes in the gene pool. The genetic health of dog breeds
opment. Breeders of small population breeds need is not a direct function of homozygosity, genetic diver-

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