What is a popular sire?
Popular sires, or male dogs that are used to produce large numbers of puppies, are one of the biggest contributors to a reduction in genetic diversity, an increase in inbreeding and elevated levels of genetic diseases within a breed. These dogs are often chosen because they have good characteristics, such as traits associated with good health. Breeders will use these dogs because they wish to improve the breed, but excessive use of any males can be detrimental to the over-all population.
A popular sire, let's call him Charlie, will pass on both good and bad genes to each of his offspring. We have previously discussed the problem of mutant genes associated with autosomal-recessive conditions, and also that every organism is a carrier for many autosomal-recessive conditions. The more puppies that Charlie helps to produce, the more his genes will be spread throughout the breed, especially if his own offspring also go on to breed, or even become popular sires themselves.
This means that there will be more dogs in the breed with the genes associated with Charlie's positive traits, but there will also be more dogs that have copies of his "silent" autosomal-recessive genes too. Remember that a dog must inherit a copy of a faulty autosomal recessive gene from both its mother and father to be affected by one of these conditions. Dogs with only one copy of a mutant gene will appear completely normal.
Many of Charlie's faulty genes may initially be rare in the breed, but as the number of his descendants increase, his genes become more and more common. Within several generations Charlie's genes could be distributed widely throughout the breed and at this point mating two dogs that are unrelated to Charlie becomes more difficult. If two dogs related to Charlie mate, then there is a chance that they both carry the same faulty autosomal recessive genes and could produce affected puppies. The more descendants that Charlie has, the higher the proportion of carriers and affected dogs there will be in the breed. It may take several years for the effects of Charlie, as a popular sire, to become apparent, but by this time his genes may already be widely distributed throughout the breed.
In addition to increasing the risk of autosomal-recessive conditions, the over use of popular sire can also impact on the size of the gene pool. Remember that the gene pool is the total amount of genetic variation within a breed, and that unless new dogs are introduced into a breed, it is likely to become smaller over time. Every litter produced by Charlie means one less opportunity for another male to reproduce. The genes from the other dogs which were not chosen for mating will become rarer and may even disappear from the gene pool entirely. The smaller the gene pool becomes, the more difficult it can be to find unrelated individuals for mating. Smaller gene pools may be more difficult to manage and may results in increasing levels of inbreeding.
To prevent the popular sire effect, stud dog owners should restrict the number of times their dog is used for stud.
The number of times a dog should be used will be dependent on the effective population size and size of the gene pool, so providing guidelines on how many puppies a stud can safely produce will be breed dependant and is difficult to estimate. We also need to consider that the popular sire effect will be most noticeable if his puppies go on to produce litters themselves. So monitoring the level of contributions (whether sires are having breeding pups) is more effective than simply monitoring the number of pups a dog sires.
However, owners of bitches looking to use a stud dog should enquire how many times a dog has been used and should avoid using known popular sires. Using a wider variety of dogs will help maintain genetic diversity.