Popular Sire Syndrome and
Concerns of Genetic Diversity

Used with permission from Today's Breeder, Nestlé Purina PetCare Company.

Most breeders aim to produce quality, healthy litters that exemplify the breed standard. What breeders may not realize is that too much breeding to one dog may give the gene pool an extraordinary dose of its genes, including any detrimental recessive genes that may not be discovered until later generations.

"Regardless of the popularity of the breed, if a large proportion are breeding to a single stud dog - called popular sire syndrome -the gene pool may drift in that dog's direction and there will be a loss of genetic diversity," says Jerold Bell, DVM, a veterinary genetic counselor and director of clinical veterinary genetics at Tufts University School of Veterinary Medicine.

"Overuse of individual dogs decreases diversity due to bottlenecking, which pushes out the influence of other quality dogs in the breeding pool," Bell says. "This can cause future breed-related genetic conditions through what is known as the founder's effect."

All genes come in pairs - one from the sire and one from the dam. If both genes are the same type, the gene pair is homozygous. If the two are different, the gene pair is heterozygous. While a dog can have a maximum of two different genes in a pair, many different genes are potentially available to be part of the pair, Bell says. Breed diversity occurs when a several genes are available for each pair.

If a gene pair is homozygous but does not contain detrimental recessive genes then there is no negative effect on a dog's health. After all, the characteristics that make a breed reproduce true to its standard are based on nonvariable or homozygous gene pairs, says Bell. On the hand, if the homozygote is made up of harmful recessive genes, it may cause small litter sizes, neonatal death, health conditions or impaired immune function.

"Here's an example," Bell says. "Seventyseven percent of the Doberman Pinscher breed has a defective gene that causes von Willebrand's disease, an autosomal recessive bleeding disorder. But Doberman breeders can test and identify carrier and affected dogs. They can decrease the frequency of the defective gene 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, breeders can help to preserve genetic diversity."

A basic principle of population genetics is that gene frequencies the percent of a particular gene, regardless whether it's normal or detrimental - do not change from the parental generation to the offspring, Bell says. Gene frequencies will remain the same regardless of the homozygosity or heterozygosity of the parents or whether the mating represents outbreeding, linebreeding or inbreeding. It is the selection of breeding stock that changes gene frequency, not the types of matings, Bell says.

"If some breeders outbreed (breeding dogs that are relatively unrelated), some linebreed (breeding dogs with a distantly related common relative) to certain dogs that they favor, while others linebreed to other dogs that they favor, then across the breed, genetic diversity is maintained," Bell says.
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's a fallacy that each dog must carry the diversity of the breed," Bell says. "Studies in genetic conservation and rare breeds have shown that this practice actually contributes to the loss of genetic diversity.

"Breeders often underestimate the amount of diversity that can be present in a breed - even one with a limited group of founders," Bell says. "A molecular study of the Chinook dog breed, which was reduced to four dogs in the 1970s, showed significant gene diversity and heterozygosity in the breed.

"By uniformly crossing all lines or families of dogs in a breed, you eliminate the differences between them and therefore the diversity between individuals," he explains. "The process of maintaining separate lines with crossing between lines and breeding back maintains diversity in the gene pool. It is the varied opinions of breeders as to what constitutes the ideal dog and their selection of breeding stock that maintain breed diversity."

Loss of genes from a breed's gene pool occurs through selection: the use and nonuse of offspring. "If a popular sire is used extensively, gene frequencies and the gene pool can shift toward his genes, limiting the gene diversity," Bell says. "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."

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