The Popular sire effect (or Popular stud/sire syndrome) occurs when a show dog with desirable physical 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 Terrier

Bedlington Terrier

The Bedlington Terrier is a breed of terrier named after the mining town of Bedlington, Northumberland in North East England.- Description :Appearance:...

s, rage syndrome

Rage Syndrome

Rage Syndrome, also known as Sudden Onset Aggression or or Avalanche of Rage Syndrome, is a serious but rare behavioural problem that has been reported most commonly in the English Cocker Spaniel but also in a variety of other dog breeds. It is often misdiagnosed as it can be confused with other...

 in English Springer Spaniel

English Springer Spaniel

The English Springer Spaniel is a breed of gun dog traditionally used for flushing and retrieving game. It is an affectionate, excitable breed with an average lifespan of twelve to fourteen years. Descended from the Norfolk or Shropshire Spaniels of the mid-19th century, the breed has diverged into...

s, and histiocytic sarcoma

Malignant histiocytosis

Malignant histiocytosis is a rare hereditary disease found in the Bernese Mountain Dog and humans, characterized by histiocytic infiltration of the lungs and lymph nodes. The liver, spleen, and central nervous system can also be affected. Histiocytes are a component of the immune system that...

 in Bernese Mountain Dog

Bernese Mountain Dog

The Bernese Mountain Dog, called in German the Berner Sennenhund, is a large breed of dog, one of the four breeds of Sennenhund-type dogs from the Swiss Alps. The name Sennenhund is derived from the German "Senne" and "hund" , as they accompanied the alpine herders and dairymen called Senn...

s. 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

Kennel club

A kennel club is an organization for canine affairs that concerns itself with the breeding, showing and promotion of more than one breed of dog...

 (KC) showed that on average 20% of dogs have recorded offspring. Golden Retriever

Golden Retriever

The Golden Retriever is a medium-sized breed of dog. They were historically developed as gundogs to retrieve shot waterfowl such as ducks and upland game birds during hunting and shooting parties. As such, they were bred to have a soft mouth to retrieve game undamaged and have an instinctive love...

s have the lowest proportion of male dogs that are sires (5%), and discounting the greyhound

Greyhound

The Greyhound is a breed of sighthound that has been primarily bred for coursing game and racing, and the breed has also recently seen a resurgence in its popularity as a pedigree show dog and family pet. It is a gentle and intelligent breed...

 (most greyhounds are not registered with the KC), the Akita Inus the highest (13%). The most popular dam (a Labrador Retriever

Labrador Retriever

The Labrador Retriever is one of several kinds of retriever, a type of gun dog. A breed characteristic is webbed paws for swimming, useful for the breed's original purpose of retrieving fishing nets. The Labrador is the most popular breed of dog by registered ownership in Canada, the United...

) 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 Dog

German Shepherd Dog

The German Shepherd Dog , also known as an Alsatian or just the German Shepherd, is a breed of large-sized dog that originated in Germany. The German Shepherd is a relatively new breed of dog, with its origin dating to 1899. As part of the Herding Group, the German Shepherd is a working dog...

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

Fédération Cynologique Internationale

Fédération Cynologique Internationale is an international federation of kennel clubs based in Thuin, Belgium. The English language translation, World Canine Organisation, is not often used.Its goals are described in Article 2 of their regulations:...

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.