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. Unfortunately many stud dogs are chosen because of their trained ability to perform and win the title FTCH. This often is not a testament to their genetic, more so to the ability of the handler and trainer that has competed with them.
Popular sires and autosomal recessive conditions
A sire, will pass on both good and bad genes to each of his offspring. Every organism is a carrier for many autosomal-recessive conditions. These are health conditions that can only affect a dog when it has two copies of a faulty gene (inherited from both its mother and father). Dogs with only one copy of the mutant gene are said to be carriers and are unlikely to show any sign of the disease, but can pass the gene on to their offspring. The mutant genes for autosomal-recessive conditions can be the most difficult to predict, because they can be passed on from generation to generation without being noticed or identified. As long as a dog also has a healthy copy of the gene to do its normal job, then the mutant gene may never be noticed. Often, there is no way to know that these mutant genes exist, or what they cause, until they are expressed in a dog with two copies.
Impact on the gene pool
In addition to increasing the risk of autosomal-recessive conditions, the over use of popular sires can also impact on the size of the gene pool. A breed’s gene pool is the total amount of genetic variation within a breed, and unless new dogs are introduced into a breed, it is likely to become smaller over time. 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 result in further increasing levels of inbreeding, like a vicious cycle.
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 actual 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 only occur 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.