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Popular Sire Syndrome and Concerns of Genetic Diversity
Popular Sire Syndrome and Concerns of Genetic Diversity
Jerold S. Bell, DVM
Tufts University School of Veterinary Medicine
(This talk was presented at the 2003 AKC Canine Health Foundation Parent Club Conference,
and the abstract published in the proceedings.)
There is a tendency for breeders to breed to the male who is the top-
winning dog. This can also occur with a popular dog that has OFA
excellent hip conformation, or has produced no epileptic offspring in
matings to epileptic dams. Regardless of the popularity of the breed, if a
large portion are breeding to a single stud dog, (the popular-sire
syndrome), the gene pool will drift in that dog's direction and there will
be a loss of genetic diversity. Too much breeding to one dog will give the
gene pool an extraordinary dose of his genes, and this will include
whatever detrimental recessives he may carry, to be uncovered in later
generations. This can cause future breed-related genetic disease through
what is known as the founder's effect. Along with the thrill of owning a
popular sire, comes your responsibility to the breed. Over time, you will
find out what detrimental genes he carries. Hopefully these will cause
minor faults, but occasionally they may cause genetic disorders. The true
measure of a conscientious breeder is how this knowledge is disseminated to the owners of the next generation.
Purebred dog breeds have closed studbooks. No new genes are available to the breed, except from infrequent mutations that
are usually not desirable. Considering a breed as a whole, genes cannot be gained through selective breeding; they can only
be lost. This has lead breeders to question whether a pure breed can go though hundreds of years of selective breeding and
still maintain its health and viability. All genes come in pairs: one from the sire and one from the dam. If both genes are of
the same type, the gene pair is homozygous. If the two are different, the gene pair is heterozygous. While each dog can have
a maximum of two different genes in a pair, many different genes are potentially available to be part of the pair. The greater
the number of genes that are available to each pair, the greater the breed diversity.
Breeders underestimate the amount of diversity that can be present in a breed; even one with a limited group of founders.
A molecular genetic study of the Chinook dog breed, which was reduced to four dogs in the 1970s, showed that there was
significant gene diversity and heterozygosity in the breed.
The studbook for the Thoroughbred horse has been closed for more than 300 years. However, researchers have found that
on average 63 percent of the variable gene pairs are heterozygous and that 4.7 genes are potentially available to each pair.
This diversity is present in spite of the fact that 95 percent of the breed traces back to a single founder male.
Some breeders express concern that inbreeding depression may affect the viability of their breed. The consequence of
inbreeding depression is not due to a general effect from a high level of homozygous gene pairs. The problem that
inbreeding depression causes in purebred populations, stems from the effects of deleterious recessive genes. When
homozygous, they cause impaired health.
Lethal recessives place a drain on the gene pool, through smaller litter size or neonatal death. Other deleterious genes can
cause disease or impair immunity. If there is no breed diversity in a gene pair, but the particular homozygote that is present
is not detrimental, there is no negative effect on health. The characteristics that make a breed reproduce true to its standard
are based on non-variable (homozygous) gene pairs.
The Doberman Pincher breed has a problem with von Willebrand's disease; an autosomal recessive bleeding disorder.
Genetic testing has found that the defective gene is present in 77 percent of Dobermans. Doberman breeders can test and
identify carrier and affected dogs. They can decrease the defective gene's frequency by breeding carriers to normal-testing
dogs and selecting quality, normal-testing offspring for breeding. By not just eliminating carriers, but replacing them with
their normal-testing offspring, genetic diversity will be preserved.
The perceived problem of a limited gene pool has caused some breeders to discourage linebreeding and promote
outbreeding in an attempt to protect genetic diversity. However, it is a fallacy that each dog must carry the diversity of the
breed. Studies in genetic conservation and rare breeds have shown that this practice actually contributes to the loss of
By uniformly crossing all “lines,” or families of dogs in a breed, you eliminate the differences between them, and therefore
the diversity between individuals. This practice in livestock breeding has significantly reduced diversity and caused the
loss of unique rare breeds. The process of maintaining separate lines, with many breeders crossing between lines and
breeding back as they see fit, maintains diversity in the gene pool. It is the varied opinion of breeders as to what constitutes
the ideal dog, and their selection of breeding stock that maintains breed diversity.
A basic tenet of population genetics is that gene frequencies do not change from the parental generation to the offspring.
The gene frequencies will remain the same regardless of the homozygosity or heterozygosity of the parents, or whether the
mating represents an instance of outbreeding, linebreeding, or inbreeding. If some breeders outbreed, and some linebreed
to certain dogs that they favor while others linebreed to other dogs that they favor, then breedwide genetic diversity is
The loss of genes from a breed's gene pool occurs through selection: the use and non-use of offspring. If a popular sire is
used extensively, gene frequencies, and the gene pool can shift towards his genes, limiting the breed's genetic diversity.
On the other hand, dogs that are poor examples of a breed should not be used simply to maintain diversity. Related dogs
with desirable qualities will maintain diversity and improve the breed.
Breeders should concentrate on selecting toward the breed standard, based on ideal temperament, performance, and
conformation, and should select against the significant breed-related health issues. If breeders continually breed healthy,
superior examples of their breed and avoid the popular-sire syndrome, the genetic health of the breed can be maintained.
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