Page 42 - Chow LIfe - 2022 01 Spring

different in each situation. If a breed or family Outbreeding proponents state that molecularly
shows higher frequency of genetic disease with identified low frequency gene variants and genetic
linebreeding, then more intense outbreeding markers should be selected for and increased in
and purposeful selection against those specific breeds (without knowing what the associated genes
diseases is necessary to diminish the causative code for). It is more likely that those low frequency
gene frequencies. If deleterious genes causing markers are the result of generations of selection
breed-related disease are old and dispersed in the against specific undesirable traits and diseases.
gene pool, then those diseases are just as likely to Heterozygosity should not be a selected goal.
be expressed with outbreeding. Direct selection Heterozygosity and homozygosity measurements
against those diseases is the only way to reduce are tools and not goals. They can be utilized in
their incidence. different situations to bring in novel genes and
Some advocate for heterozygosity of major traits, or to create uniformity of existing genes and
histocompatibility complex (MHC) genes that traits. Increased homozygosity should also not be a
regulate the immune system. However, all peer- breeding goal. Inbreeding coefficients should only
reviewed published studies on immune-related, increase due to purposeful linebreeding for quality
immune-mediated, and auto-immune diseases and health.
identify specific MHC liability genes, and not Homozygosity measurements are not a
general MHC homozygosity or diversity.6-9 measurement of individual or population health
Breed genetic diversity involves selecting or vitality. The only way to measure breed health
individuals for breeding from the breadth of is through breed health surveys that document
the gene pool, not the types of matings that clinical disease and reproduction parameters.
they are involved in. With an expanding breed Homozygosity is not inherently correlated to
population, the average relationship (IC based on a impaired genetic health and does not need to be
set number of generations) between individuals in artificially controlled. Managing breeds requires
one generation will be lower than in the previous breed conservation efforts, not species survival
generation. This is why (in the absence of popular plans.10
sire effect or other diversity limiting parameters)
generational inbreeding coefficients over time Practical aspects of gene pool diversity
go down in well managed breeds. However, the Based on AKC statistics, on average only 10.4%
breeders of these breeds are all doing different (for populous breeds) to 13.9% (for smaller
types of matings (outbreedings, linebreedings, population breeds) of dogs within a breed
etc.) based on their needs and their selection reproduce to create the next generation of dogs.
preferences to improve the health and quality of This represents a genetic bottleneck with each
their dogs. generation in every purebred dog population. It
Diversity breeder enthusiasts look at the graph of emphasizes the fact that breeders must utilize the
a breed's average ICs over time and say, “Well if breadth of the gene pool background in selecting
decreasing average ICs represent a healthy breed dogs for breeding, and judiciously select dogs with
then why not just plan matings with lower ICs?” It the best health and quality.
sounds reasonable. Genetic diversity also exists in dogs from the same

However, the impact of everyone outbreeding breed on different continents. Molecular genetic
studies show that breed subpopulations diverge
causes the homogenization of breeds, so differences and can be differentiated, even though all members
between "lines" disappear. If outbreeding between of the breed descended from the same breed
the two most unrelated dogs, their offspring make founders. While there may be subtle differences
those lines related. The next mating must be to in selection for conformation between continents/
a dog unrelated to the two original lines and kennel clubs, this genetic diversity can be utilized
now these three lines are related in the offspring. in matings.
Continued matings in additional generations to
unrelated dogs becomes more difficult as dogs Frozen semen from quality dogs several
become homogenized and related to each other. generations back are another source of genetic
If everyone outbreeds, it disrupts the ancestral diversity. Many breed clubs have created club-
pedigree structure of breeds that was based on owned frozen semen repositories for breeders who
selection. It removes the genetic differences do not wish to retain semen or continue to pay for
between dogs that are necessary for genetic their storage. Knowledge of the dog’s health and
improvement through selective breeding. qualities are important in their use. DNA testing
can be performed on a semen sample.

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