In a recent post, I identified three things breeders can do that will improve the health of purebred dogs (Three key strategies to reduce genetic disorders in dogs). One of these was to prevent a few individual dogs from making a disproportionate contribution to the gene pool by having more than their fair share of offspring. This, of course, is the Popular Sire - that top-ranked dog that you're convinced has all the qualities you would dearly love to see in your next litter. Everybody wants a piece of him, and convinced that he has so much to offer the breed that it would be wrong not to use him, breeders rush to get in his date book and the proud owner basks in the glow of his celebrity.
But popular sires destroy gene pools. How could such a beautiful animal be so bad for the breed?
This was first published in December 2013, but if you missed it then or are new to the fancy, here's the story of Hank and his unfortunate genetic legacy.
The Popularity of Popular Sires
Even a century ago Williams Haynes (1915) was writing about the "Effect of the popular sire", noting that in three terrier breeds that he examined - Irish Terriers, Scottish Terriers, and Fox Terriers - about 40% of the puppies were sired by only 20% of the sires. Back then, "popularity" was quite different than now - his "prolific" dogs sired 5-7 litters, which would be completely unremarkable today. And surprisingly, Haynes thought that popular sires actually benefitted the breed by contributing to the preservation of variability in type.
Superficially it might appear that if approximately 40% of the puppies each year are sired by but 20% of the stud dogs this would eventually result in the greatest uniformity of type. The selected sires are all to a greater or lesser degree exceptional animals, but they are not selected by any uniform system. Most of them excel in some particular physical point, but they do not excel in the same points or in the same degree, nor even, in some cases, in the same direction. Here the personal equation, the ideals of different breeders, is at work, and the result is that since a few males not themselves of uniform type sire a greater-than-average number of offspring they disturb the race average of the following generation and introduce abnormal amounts of variation. The fact therefore, that artificial selection gives to certain selected, but not uniform, males an undue preponderance of influence must always keep the type of domestic animals in an unstable state. This seems to me an important factor in the great variability always noted among domesticated breeds.
Haynes thought popular sires were a good thing, because he thought they were sufficiently different from each other that they prevented the breed from becoming too "uniform". How then did the popular sire go from contributing to the quality of the gene pool in 1915, to the source of a problem to be avoided by breeders 100 years later? What is this "syndrome" that today's geneticists are so concerned about?
Breaking Bad: DNA
To understand the problem, you must understand a bit of genetics. You probably know about mutations - bits of DNA that are not replicated perfectly or are perhaps damaged by some environmental toxin. If the mutation is dominant and affects some vital process, it is removed from the gene pool by natural selection when that individual fails to pass its genes on to the next generation successfully. But many mutations have no ill effects because their paired, dominant allele functions normally. These "recessive" mutations are silent in the genome and can be passed to the next generation the same as any other gene, and as long as the offspring has a copy of a normal allele the mutation remains silent. The mutation becomes a problem when an individual inherits two copies so is homozygous at that locus. Without at least one copy of the normal, unmutated allele, the gene does not function properly, and the consequence can range from something relatively trivial (e.g., a different eye color, or slightly shorter legs) to the catastrophic (e.g, blindness, disruption of a critical biochemical pathway, cancer).
Mutations happen all the time. The ones with immediate ill effects are removed from the gene pool by natural selection, while the recessive, silent ones remain in the genome as the "genetic load". Every dog - in fact, every organism - has its own unique collection of damaged alleles that causes no harm as long as there is also a copy of a normal allele of each that can do the job it is supposed to.
A Star is Born
Now consider what happens in a population of purebred dogs. Let's pretend that this cute collection of dogs represents your breed, with the phenotypic variations among them representing the nuances of type that would be obvious to a serious breeder. We've given each dog a (typographic) recessive mutation, a bit of DNA damage that is not expressed so it has no detrimental effect on the dog. If each dog in our population has a litter of puppies this year, the frequencies of these various alleles in the population will stay about the same in the next generation.
But what happens if one of these dogs wins big at an important event and becomes a star? If it's a bitch, she will have a litter of much sought-after puppies, and it will probably be at least a year before she is bred again.
If our star is male (let's call him "Hank"), he will be bred many times and produce dozens (or more!) puppies in a single year. Hank will pass half of his genes, both good and bad, to each of his offspring, so many copies of his recessive, silent mutations get distributed in his puppies.
As long as Hank's deleterious mutations are paired with a normal allele in his puppies, they are not expressed and cause no ill effects. But if you could view the gene pool of the breed in the new generation, you would see that now it is markedly different.
Hank's mutation has in just a single generation gone from being rare to common, and now lurks silently in the genomes of dozens of his offspring. In this generation, noone is any the wiser. The prized puppies that carry their sire's recessive mutation will appear to be no different than the ones that don't.
The Next Generation...
But in the next generation we start to see the first hint of trouble. Perhaps there were a few half-sib matings, or father-to-daughter, and some puppies are produced that are homozygous for Hank's mutation. Perhaps the mutation is lethal and these are stillborn pups, or maybe the puppies are born with a disease. But the breeders will be mystified - they have never had this problem in their line, or even in the breed, so maybe it's just bad luck? Nobody can see yet that this is just the tip of the iceberg.
Every litter produced by this popular sire is one less reproductive opportunity for any of the other potential sires in the breed, so the frequency of genes carried by those unused sires will decline in the population. At the same time, multiple bitches are producing puppies sired by Hank that will be half-sibs to the dozens of other puppies in their generation. The temptation to capture a bit more of that popular sire's star qualities will probably result in a few line breedings that will put carrier with carrier.
This is about the time breeders begin to notice that there is a "problem" in the breed. It won't take a pedigree sleuth to trace the growing population of affected dogs back to Hank, our popular sire who will now be blamed for introducing this new disease into the breed. Geneticists will be called in to hunt for the defective bit of Hank's DNA and to develop a reliable test. Then breeders will begin the mission of trying to eliminate Hank's formerly valuable genes from the gene pool, with proportional collateral damage to the genetic legacy of all of the bitches he was bred to. The genetic carnage resulting from attempts to purify the breed of the unfortunate mutation will continue for generations. The ultimate damage to the gene pool can be catastrophic.
The Unfortunate Legacy of the Popular Sire
The really unfortunate thing about the Popular Sire is that the negative genetic consquences of his popularity don't begin to manifest for generations, by which time the breed already has a really significant problem.
The large number of breed-specific disorders known to be caused by a single recessive gene (175 as of this writing; OMIA) is testimony to the prevalence of the problem (indeed, some breeds now suffer from multiple recessive genetic disorders).
Leroy (2011) has identified popular sires as the single most important contributor to the dissemination of genetic diseases in purebred dogs. Recognizing this, the FCI has issued a recommendation to breeders that no dog should have more offspring (presumably in its lifetime) than equivalent to 5% of the number of puppies registered in the breed during a five-year period, and a number of national kennel clubs have followed suit (e.g., Finland). But without cooperation of breed clubs, or in the absence of some authority that would oversee registrations and be in a position to police such a breeding restriction, it is hard to see how such a recommendation would have any effect at all on current breeding practices. (Which 5-year period? Which population of dogs - the worldwide breed, or just the dogs in your country? Who does the counting - the owner of the sire, the owner of the bitch, the breed club, the kennel club??).
The only people benefitting from the explosion of breed-specific genetic disorders are the molecular geneticists, who have discovered dogs as an ideal research animal because many of the same disorders occur in humans (Ostrander 2012). But as useful and fascinating as dogs might be for their research, I suspect all would prefer to see dogs that are free of genetic disease, for they have so much more to offer in the family home than in the lab.
- Dobson, JM. 2013. Breed-predispositions to cancer in pedigree dogs. ISRN Veterinary Science 2013: (doi: 10.1155/2013/941275)
- Duffy, DL, Y Hsu, JA Serpell. 2008. Breed differences in canine aggression. Applied Animal Behaviour Science 114: 441-460.
- Haynes, W. 1915. Effect of the popular sire. Journal of Heredity 6: 494-496.
- Leroy, G. 2011. Genetic diversity, inbreeding and breeding practices in dogs: results from pedigree analyses. Veterinary Journal 189: 177-182.
- Leroy, G & X. Rognon. 2012. Assessing the impact of breeding strategies on inherited disorders and genetic diversity in dogs. Veterinary Journal 194:343-348.
- Moore, PF. 1984. Systemic histiocytosis of Bernese Mountain Dogs. Veterinary Pathology 21: 554-563.
- Moore, PF & A Rosin. 1986. Malignant histiocytosis of Bernese Mountain Dogs. Veterinary Pathology 23: 1-10.
- Ostrander, EA. 2012. Both ends of the leash- the human links to good dogs with bad genes. New England Journal of Medicine 367: 636-346.
- Reisner, IR. & KA Houpt. 2005. National survey of owner-directed aggression in English Springer Spaniels. Journal of the American Veterinary Medical Association 10: 1594-1603.
- Wellman, R. & J. Bennewitz. 2011. Identification and characterization of hierarchical structures in dog breeding schemes, a novel method applied to the Norfolk terrier. Journal of Animal Science 89: 3846-3858.
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