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The popular sire effect (or popular stud/sire syndrome) occurs when an animal with desirable attributes is bred repeatedly. In dog breeding, a male dog that wins respected competitions becomes highly sought after, as breeders believe the sire possesses the genes necessary to produce champions. However, the popular sire effect is not just down to wanting to produce a champion. For example, in Staffordshire Bull Terriers there are several popular sires who are used by breeders to produce specific colours that are not favoured in the show ring. The popular sire is often bred extensively with many females. This can cause undetected, undesirable genetic traits in the stud to spread rapidly within the
gene pool The gene pool is the set of all genes, or genetic information, in any population, usually of a particular species. Description A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can survi ...
. It can also reduce
genetic diversity Genetic diversity is the total number of genetic characteristics in the genetic makeup of a species. It ranges widely, from the number of species to differences within species, and can be correlated to the span of survival for a species. It is d ...
by the exclusion of other males. While a popular stud can sire a large number of litters, the effect of a popular dam is more limited. The constraints of the female reproductive cycle requires dams to have several months in between each litter so, even at capacity, a dam would not be able to parent as many offspring as a sire; furthermore, a dam bred repeatedly may experience a sharp decline in her litter count due to a weaker body caused by too many consecutive pregnancies.


Genetic consequences

Genetic diversity in a population is dependent on the cumulative effects of
mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, ...
,
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the Heredity, heritable traits characteristic of a population over generation ...
, and population size over many generations.Frankham, Richard; Ballou, Jonathan; & Briscoe, David. ''Introduction to Conservation Genetics''. Cambridge: Cambridge University Press, 2009. Print. The
effective population size The effective population size (''N'e'') is the size of an idealised population that would experience the same rate of genetic drift as the real population. Idealised populations are those following simple one- locus models that comply with ass ...
(''Ne'') is the number of individuals in a given population that are able to reproduce and contribute offspring to the next generation. Genetic diversity decays over generations at a rate that is inversely related to the effective population size. In small populations, a major factor that causes a loss of genetic diversity is
inbreeding Inbreeding is the production of offspring from the mating or breeding of individuals or organisms that are closely genetic distance, related genetically. By analogy, the term is used in human reproduction, but more commonly refers to the genet ...
. Intentionally breeding for desirable attributes in a species is known as
selective breeding Selective breeding (also called artificial selection) is the process by which humans use animal breeding and plant breeding to selectively develop particular phenotypic traits (characteristics) by choosing which typically animal or plant m ...
. This process artificially limits the size of the population pool, thereby increasing the rates of inbreeding as well as
inbreeding depression Inbreeding depression is the reduced biological fitness caused by loss of genetic diversity as a consequence of inbreeding, the breeding of individuals closely related genetically. This loss of genetic diversity results from small population siz ...
, and decreasing a species biological fitness. Deleterious mutations in dominant traits are removed from the gene pool by natural selection when the affected individual fails to successfully pass the gene onto the next generation. Recessive mutations, on the other hand, are only expressed and selected against in
homozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mos ...
recessive individuals; thus, they can be silently passed onto the next generation in
heterozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mos ...
individuals. Excessive breeding of any one dog will cause an extraordinary dose of his genes, including any deleterious
recessive In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and ...
alleles he may carry, to be contributed to the gene pool. The accumulation of these recessive mutations in the reproducing individuals of a population can cause breed-related diseases in future generations through a phenomenon known as the
founder effect In population genetics, the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population. It was first fully outlined by Ernst Mayr in 1942, us ...
.Bell, Jerold S. DVM. “Popular Sire Syndrome and Concerns of Genetic Diversity.” Tufts University Cummings School of Veterinary Medicine. 2003 AKCCHF Health Conference. Diseases attributed to the popular sire effect include copper toxicosis in
Bedlington Terrier The Bedlington Terrier is a dog breed, breed of small dog named after the mining town of Bedlington, Northumberland in North East England. Originally bred to hunt, the Bedlington Terrier has since been used in dog racing, numerous List of dog s ...
s, rage syndrome in
English Springer Spaniel The English Springer Spaniel is a breed of gun dog in the Spaniel group traditionally used for flushing and retrieving game. They are descended from the Norfolk or Shropshire Spaniels of the mid-19th century; the breed has diverged into sepa ...
s, and histiocytic sarcoma in Bernese Mountain Dogs. Pedigree analysis of Bernese Mountain Dogs in France showed that only 5.5% of males and 13.2% of females are used for reproduction each generation, with 0.78% of males and 3% of females producing more than 50% of the next generation. Because the offspring and grand-offspring of sires turn out good, breeders often breed them to each other and conduct additional backcrosses for generations. Sometimes a sire will be bred so extensively that, decades later, breeders may not be able to tell just how closely related their dogs are, as the sire's history has run off their pedigree. This happened with the
Australian Shepherd The Australian Shepherd is a breed of herding dog from the United States. The name of the breed is technically a misnomer, as it was developed in California in the 19th century. It is believed to have its origins in sheepdog breeds from northwe ...
, with most show-line Aussies tracing back, repeatedly, to two full brothers: Wildhagen's Dutchman of Flintridge and Fieldmaster of Flintridge. In Quarter Horses, hyperkalemic periodic paralysis (HYPP), an autosomal dominant disorder of potassium regulation, can be linked back to a single Quarter Horse sire called Impressive. Also known as Impressive syndrome, HYPP causes heavy muscling in carrier horses, which is a desirable trait in the show ring. However, homozygotes can suffer from severe disease, which manifests as attacks of muscle twitching, weakness, collapse, and death. The disease became widespread when Quarter Horse breeders began selecting for muscling; by 2003, Impressive had over 55,000 descendants. Quarter Horses are now routinely screened for the HYPP gene prior to breeding. A study of 10 breeds from the UK's Kennel Club (KC) showed that on average 20% of dogs have recorded offspring. Golden Retrievers have the lowest proportion of male dogs that are sires (5%), and discounting the
greyhound The English Greyhound, or simply the Greyhound, is a dog breed, breed of dog, a sighthound which has been bred for coursing, greyhound racing and hunting. Some are kept as show dogs or pets. Greyhounds are defined as a tall, muscular, smooth-c ...
(most greyhounds are not registered with the KC), the Akita Inus the highest (13%). A study of three varieties of terrier in 1914 indicates that about 8% of stud dogs (21 individuals) account for 23% of puppies (451 individuals), an average of 21.5 individuals per stud. In comparison, the UK study showed that 5% of Labrador males and 10% of Golden Retrievers sired more than 100 individuals each. The most popular dam (a
Labrador Retriever The Labrador Retriever or simply Labrador or Lab is a British list of dog breeds, breed of water dog retriever gun dog. It was developed in the United Kingdom from St. John's water dogs imported from the Newfoundland Colony, colony of Newfoun ...
) carried 72 offspring, while the most popular stud (an English Springer Spaniel) sired 2,538 offspring. In many breeds, more than 90% of unique genetic variants are lost over six generations.


Mitigation methods

The
Association of Zoos and Aquariums The Association of Zoos and Aquariums (AZA), originally the American Association of Zoological Parks and Aquariums (AAZPA), is an American 501(c)(3) nonprofit organization founded in 1924 and dedicated to the advancement of zoos and public aqu ...
maintains studbooks for their captive species, which contain the genetic and demographic identity of a population and can serve as valuable tools for tracking and managing each individual in an '' ex situ'' population. Studbooks and other forms of pedigree tracking can be useful tools to combat the problem of the popular sire effect and advise breeding decisions that will enhance genetic diversity. The Doberman Pinscher breed is overwhelmingly affected by
von Willebrand disease Von Willebrand disease (VWD) is the most common heredity, hereditary coagulopathy, blood-clotting disorder in humans. An acquired form can sometimes result from other medical conditions. It arises from a deficiency in the quality or quantity of ...
, an autosomal recessive bleeding disorder;
genetic testing Genetic testing, also known as DNA testing, is used to identify changes in DNA sequence or chromosome structure. Genetic testing can also include measuring the results of genetic changes, such as RNA analysis as an output of gene expression, or ...
has revealed that the detrimental gene is present in 77% of Dobermans. To decrease the frequency of the defective gene, Doberman breeders can breed carriers of the gene with unaffected individuals, and subsequently breed the non- carrier offspring. In any population that is overly-inbred and at risk, breeders should run pedigree analyses and genetic testing for deleterious alleles in each individual before making their breeding decisions. Actions to limit the use of popular sires have been recommended to reduce the loss of genetic diversity in individual breeds. Such limits are in effect in
German Shepherd Dog The German Shepherd, also known in Britain as an Alsatian, is a German breed of working dog of medium to large size. The breed was developed by Max von Stephanitz using various traditional German herding dogs from 1899. It was originally b ...
s in Germany. The Norwegian Kennel Club recommends that no individual dog should have more offspring than the equivalent of 5% of the number of puppies registered in its breed during a five-year period. The
Fédération Cynologique Internationale The (; FCI; ) is the largest international federation of national kennel clubs and purebred registries. The FCI is based in Thuin, Belgium and has 98 members and contract partners (one from each country). History The FCI was founded in 1911 un ...
adopted the same recommendation in March 2010.


See also

* Complex vertebral malformation, a widespread lethal genetic disease in Holstein cattle traced back to popular sire Carlin-M Ivanhoe Bell. *
Founder effect In population genetics, the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population. It was first fully outlined by Ernst Mayr in 1942, us ...
*
Population bottleneck A population bottleneck or genetic bottleneck is a sharp reduction in the size of a population due to environmental events such as famines, earthquakes, floods, fires, disease, and droughts; or human activities such as genocide, speciocide, wid ...
*
Artificial selection Selective breeding (also called artificial selection) is the process by which humans use animal breeding and plant breeding to selectively develop particular phenotypic traits (characteristics) by choosing which typically animal or plant ...


References

{{reflist Population genetics