Page 41 - Chow LIfe - 2022 01 Spring

to homozygosity and will eventually go extinct. of selective breeding. Mars Wisdom Panel
However in reality, these populations are robust computations show that mixed-breed dogs have on
and expanding because deleterious genes are average 53% homozygosity and purebred dogs 63%
not at a high frequency. This is not to say that homozygosity. This increase in homozygosity is
homozygosity should be a goal of breeding. It does not deleterious to breeds unless it causes increased
show that homozygosity by itself does not cause expression of genetic disease.
disease and poor health, and is not necessarily
deleterious to a population. What is deleterious is Endangered species survival is based solely on
producing viable offspring. This underscores
the accumulation of disease-associated genes. the importance of SSP programs to prevent the
Natural selection requires large populations and homozygous expression of disease-associated
genetic drift to improve species. With artificial recessive genes. Published metadata from Mars
selection, breeds do not require a large population show that mixed-breed dogs carry statistically
size for genetic improvement. Few dog breeds fulfill higher frequencies of 152 testable disease-
the population thresholds determined for natural associated genes than the combined tested
species to be able to survive. However, few breeds purebred dog populations.4 It is the population
exhibit inbreeding depression requiring SSP-like diversity of mixed breed dogs that reduces the
rescue programs. Most dog breeds are robust, expression of these recessive diseases. Linebreeding
and only require continued reproduction and in mixed breed dogs would be expected to produce
selection for quality and health. Breeds with small more recessive genetic disease than it does in
populations look like populous breeds did earlier purebred dogs. (Common complexly inherited
and just need proper selection and population genetic diseases are seen routinely in mixed breed
expansion. dogs.) Selection for health occurs in purebred
What is homozygosity, and what does it tell you? dog matings. Selection for health diminishes
the frequencies of disease associated genes and
Homozygosity is the pairing of “like” genes in gene increases the homozygosity of health-related genes.
pairs. All genes come in pairs – one from the sire
and one from the dam. If the sire and dam share Diversity Breeding
a common ancestor, then the same genes can be Diversity breeding enthusiasts recommend SSP-
passed down through both parents and pair up in type mating plans and only outbreeding (matings
the offspring. The effect of homozygosity is that it between dogs less related than the average in the
causes uniform expression (I.e., trait, characteristic, population). What does outbreeding do to breed
or disease) in all individuals inheriting the genetic diversity? If you take a group of dogs and
homozygous gene pair. There are positive only breed them to the least related in the group,
genes that you want to select for (and create you will have lower homozygosity. If you take
homozygosity), as well as deleterious or disease- the same group of dogs and do linebreedings
causing genes that you want to select against. (matings between dogs more related than the
To understand what homozygosity measurements average in the population) you will have higher
homozygosity. Have you changed the population or
represent, we must understand how homozygosity the genetic diversity of the breed? No. It is the same
purposefully develops in a breed. Purebred dog group of dogs with the same genes. Breeding for
breeds were created through artificial selection for heterozygosity does not improve or change genetic
specific tasks or traits. Through constant selection diversity. It only masks the expression of recessive
towards these breeding goals, breed characteristics or additive genes; both positive and deleterious.
reproduce uniformly through generations.
For a breed to reproduce uniformly, it requires Does breeding for heterozygosity improve breed
health? Embark studied data from the Morris
homozygosity of genes. The genes that cause Animal Foundation Golden Retriever Lifetime
mammals to be mammals are homozygous, the Study and found that on average, a 10% increase in
genes that cause dogs to be dogs are homozygous, inbreeding coefficient of the mother (not the litter
and the genes that cause a Gordon Setter to be a IC, which was not studied) decreased litter size by
Gordon Setter are homozygous. 1 puppy.5 This puppy loss would be expected to be
It does not take intense linebreeding to create the result of homozygosity of embryologically fatal
homozygosity. Constant selection for certain traits recessive genes.
will increase the frequency and homozygosity of Every breed and breed family has different
their causative genes. Creating homozygosity of frequencies of deleterious recessive and
genes for desirable traits and against disease- additive genes in their background. The
associated genes is the measurable result effects of linebreeding are going to be

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