What’s the relationship between inbreeding dogs and genetic diseases in dog breeding? Because of the high degree of inbreeding inherent in many purebred dog lines, a wide range of genetic defects are over-represented in such dogs, compared to mixed breed mongrels. The overwhelming majority of these defects are recessively inherited. It is absolutely essential to do a little research on the breeds you are considering working with to ascertain the most common issues in that breed – every breed will have something lurking in the genetic pool. However, some are worse afflicted than others, so do your homework first! Some of these inherited defects will be lethal before the age of breeding (e.g. portavenal liver shunt) but many only manifest as problems in older dogs.
Normally, problems only arise when two dogs that both happen to carry the same recessive genetic defect are mated together. By the simple laws of genetics (presented in the previous section) the offspring of such a mating where neither parent actually expresses the fault (i.e. are heterozygous for it) have a 25% chance of suffering from the defect. If one parent actually expresses the fault (e.g. hip dysplasia, heart problems, cataracts) then the chance of having similarly affected puppies rises to 50%. If both parents suffer from the fault (and are therefore genetically homozygous for it), then 100% of the puppies will be affected.
Particular lines within a breed drawn from a limited genetic pool of dogs will have their own particular genetic defects. This naturally arises over a few generations whenever breeders limit their choice of stud dogs. Many breeders seem oblivious to the dangers of this practice. Motivated by trying to produce the perfect dog that conforms exactly to the associated breed standards, breeders deliberately breed outstanding show animals back to their close relatives in order to both stabilize and stamp their line with its special characteristics. They call this practice line breeding, but it is really inbreeding.
Inbreeding/line breeding is dangerous and risky, and has no place in a dog breeding enterprise geared towards producing viable, vigorous puppies for the pet market from healthy, fertile, trouble-free stock.
The first thing to suffer in any animal with closely related parentage is what is called in genetics “vigor”. Vigor refers to the fecundity (fertility), health and life expectancy of the animal. Inbred animals are more prone to poor reproductive performance, have smaller litters and are simply not as robust and long-lived as out-bred animals. When you cross totally unrelated individuals, on the other hand, the progeny are imparted with what is called “hybrid vigor”, and will show high reproductive performance, strength, resistance to illness, and overall health and vitality. In accord with the laws of genetics, the progeny have a very low chance of being homozygous for any deleterious traits, and thus tend to exhibit the strengths of both parents. This principle is widely used in livestock where the progeny are bred for meat production under tough conditions – in cattle, for example, Asian breed bulls (eg the hump-backed Brahman) will be mated over a very distantly related breed such as British breed cows (eg Angus) to produce a tough “Brangus” hybrid that will do well under a range of environments.
Line-breeding is often hotly defended by breeders. I suspect that such breeders have had to resort to uncomfortably close breeding because they are too ornery to get along enough together to access each other’s bloodlines!
Once you have established a connection with a breeder you can work with, ask for their advice about every aspect of breeding your chosen type, including where you might source dogs suitable to mate with their stock. You will find that many breeders will malign the stock of other breeders. There is a lot of jealously between breeders, especially those who show their animals and compete with each other.
Some will happily tell you about the congenital defects lurking in the lines of their competitors’ dogs. Truth be told, all pedigree breeds, simply because they are pedigree (and therefore have a limited genetic pool), will have some disease or other in their lines. It goes with the territory! Do your research and find out the most common problems in the breed you fancy. For some genetic disorders, testing of the parent stock is a good screen for possible problems in their progeny (e.g. hip dysplasia). For many others, however, it is not. Most congenital disorders are either expressed in the dog or not.
Just because a dog is not affected, doesn’t mean that its progeny won’t be. The only way to reduce the risk is to stay as far away from inbreeding as you can, since when dogs carrying the same genetic weakness (highly likely in closely related animals) are mated together, the chance that they will have offspring that actually expresses and therefore suffers from the weakness is very high. You don’t want people coming back to you for a refund because the pup you sold them turned out to be defective, do you? And a carefully built reputation can be dashed instantly by such an occurrence!
Line breeding does produce some “outstanding” show animals, but just as many defective “duds” not suited to any purpose, least of all as innocent people’s beloved pets. Line breeding is only appropriate if coupled with heavy selection pressure and under very limited circumstances. In production animals such as livestock only a carefully selected few progeny of such matings are retained – the rest are culled, and for good reason.
The problem of inbreeding also commonly occurs within a specific geographic area where the number of purebred dogs is very limited. Sometimes the number of registered animals of certain breeds within particular countries is so low that it is almost impossible for breeders to avoid mating close relatives.
Another common scenario is what is termed “popular sire syndrome”. This may occur if a breeder imports an outstandingly successful show dog that is then made available for use by other breeders. With artificial insemination and semen storage technology, such sires can father a very large number of litters, and potentially make a significant impact on the gene pool, particularly if it is small to begin with. The implications are that:
- It may be soon become difficult to source dogs that are unrelated to this super-stud.
- The super-stud or its progeny may go on to manifest a genetic problem after it has already left a major impact on the gene pool.
Within a particular family line many individuals will be extremely likely to harbor the same recessive defect. So it is most important to realize that the chances of both parents carrying the same genetic fault increases dramatically when closely related dogs are bred together. The moral of the story for you is to:
- Find out the problems commonly inherent in your breed and whether it is possible to screen puppies or their parents for the disorder before you buy.
- Choose your breeders from lines that are as unrelated as possible. This may require sourcing your females, for example, locally, and selecting a male from a distant location such as interstate.
- Choose breeding stock that is as outbred as possible (the same dog does not appear more than once in its pedigree). Inbred dogs not only have a high chance of carrying defects, but also suffer from the classic genetic phenomenon of “inbreeding depression”.
“Inbreeding depression” refers to an overall deterioration in health, longevity, vitality and fecundity (i.e fertility) that is proportional to the degree of inbreeding of the animal. Conversely, when animals from totally unrelated genetic pools are mated together, the progeny enjoy “hybrid vigor” – they are healthier, live longer and are more fertile. This is one reason why mongrels are generally more vigorous and healthy, breed easily, and have less need of veterinarians than purebreds.
There are a range of inherited genetic disorders common to particular dog breeds that you should consider. However, this is just an overview. I urge you to do some in-depth study yourself on any breeds you are considering and ascertain their specific issues so that you are well informed and can screen prospective breeding dogs and breeders accordingly.
Hip and Elbow Dysplasia
Hip and elbow disease can cause both pain and disability when the dog becomes lame. Hip dysplasia refers to abnormal growth or deformed structure, leading to malformation of the hip joint. Clinically affected dogs suffer from joint looseness, excessive joint wear and tear, and inflammation, leading to painful arthritis. As hip dysplasia is primarily a bone growth disorder, it is most prevalent in large and fast-growing dogs.
Hip dysplasia usually develops in younger dogs, although some dogs are not symptomatic until they are fully mature, or even seniors. It is a painful, crippling disease that causes a dog’s hips to weaken and degenerate. Because of their greater size, hip dysplasia is more common in males than females and some breeds are more prone to the disease than others.
As the problem is primarily genetic in nature, voluntary testing schemes run by the Kennel Club and many other leading canine organizations enable breeders to have their dogs’ hips X-rayed and scored for hip and elbow dysplasia, so that they can avoid breeding from unhealthy dogs. Hip registration certification, such as offered by the Orthopedic Foundation of America (OFA), is only possible upon examination of radiographs (x-rays) taken after the dog is two years old. Dogs less than two years of age can have preliminary x-rays to establish the absence of hip dysplasia, but cannot be certified free until they reach two years of age.
Hip scores range from 0 (perfect hips) to 106. A total score of 20 indicates a ‘mild degree of hip dysplasia’ and 64 would indicate a ‘gross degree of hip dysplasia.’ While there is usually a wide range of scores within a particular breed, some degree of hip dysplasia is evident in 21% of breeds tested by such schemes, meaning a ‘breed mean score’ of 20 or more. These breeds include:
- Labrador Retriever
- Golden Retriever
- German Shepherd
- Pembroke Corgi
- Sussex Spaniel
Breeds that are known to have a higher incidence of elbow dysplasia include:
- Basset Hounds
- Bernese Mountain Dogs
- English Mastiffs
- German Shepherds
- Golden Retrievers
- Great Danes
- Irish Wolfhounds
- Labrador Retrievers
The sensible approach for those of you contemplating such breeds is to ensure that you select stock whose parents have been shown by the appropriate scoring examinations to have healthy hip and elbow joints. Look for pups that have parents with “excellent” ratings, and you will be well on your way to avoiding problems.
- Heart Disease – Some breeds are highly predisposed to developing life-threatening heart problems including dilated cardiomyopathy (particularly the Dobermann and Boxer), coronary artery vasculitis (especially Beagles), and aortic stenosis (Boxer, Bulldog, Golden Retriever and Rottweiller).
- Eye Problems – About 30% of all breeds are prone to a range of inherited eye or eyelid conditions, some painful or leading to partial or total blindness. These include cataracts, glaucoma, and retinal defects including retinal detachment and progressive retinal atrophy, as well as eyelid and eyelash conditions that can cause inflammation and pain. Eye Schemes exists to screen dogs for specific eye abnormalities and diseases that are either known or suspected to be inherited in breeds.
- Glaucoma – which is associated with an increase in pressure inside the eye – results in progressive loss of vision. Affected breeds include:
- Basset Hound
- Cocker Spaniel
- Welsh Springer Spaniel
- Flat-coated retriever
- Siberian Husky
- Progressive Retinal Atrophy (PRA), which causes blindness, is prevalent in:
- Rough and the Smooth Collie
- Border Collie
- Miniature Short-Haired Dachshund
- Irish Setter
- Golden Retriever
- Labrador Retriever
- Shetland Sheepdog
- American CockerSpaniel
- Springer Spaniel
- Chesapeake Bay Retriever
- Toy and Miniature Poodle
- Norwegian Elkhound
- Miniature Schnauzer
- Welsh Corgi
In all breeds studied to date, PRA is believed to be a three-gene recessively inherited trait, so for an animal to develop PRA both parents have to carry the recessive PRA gene. Thus many dogs with normal vision and no signs of PRA may be carriers. It is impossible to determine by physical exam if an animal is a carrier, or a normal, non-carrier, as they both have the came phenotype (same appearance). Only through breeding trials can carrier dogs be identified. As this is impractical, PRA will prove very difficult to eliminate from purebred dogs.
Night blindness is an early indicator of the disease which is not painful. Most affected (blind) dogs can function well in a familiar environment. PRA can be detected by an electroretinogram before any blindness becomes apparent. Though it may develop much earlier in some, most affected breeds don’t show ophthalmoscopic lesions until they are four to seven years of age.
- Some breeds may also have a high inherited risk of developing cataracts. However, it is quite normal for older dogs (10 years or more) to develop “senile cataracts” due to their age – much the same as occurs in humans. Inherited cataracts usually develop in both eyes so if a dog only has one cataract it is more likely to be due to trauma (injury) than inheritance.