The popular sire effect (or popular stud/sire syndrome) occurs when an animal with desirable attributes is bred repeatedly. This can cause unknown undesirable genetic traits in the stud to spread rapidly within the gene pool. It can also reduce genetic diversity 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.
Diseases attributed to the effect include copper toxicosis in Bedlington Terriers, rage syndrome in English Springer Spaniels, 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. 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 (most greyhounds are not registered with the KC), the Akita Inus the highest (13%). The most popular dam (a Labrador Retriever) carried 72 offspring, while the most popular stud (an English Springer Spaniel) sired 2538 offspring. In many breeds, more than 90% of unique genetic variants are lost over six generations.
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 Dogs 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 adopted the same recommendation in March 2010.
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.