I had a disturbing, but revealing, conversation with a stranger a few weeks ago. She had a beautiful Border collie, and was planning on breeding from her. Collies are common pets here in New Zealand, and are generally healthy, but as with all dog breeds they can suffer from and carry genetic defects, such as hip dysplasia. We got talking about breeding and genetics, but the dog's owner wasn't planning on having any testing done on either of the two prospective parent dogs: instead, she was planning on crossbreeding to minimise any chances of disease.
The idea that crossbreeds are healthier than purebred dogs is not new. In a way, the reasoning is sound: outbreeding introduces more genetic variation than inbreeding. Crossbred dogs can show hybrid vigour (heterosis), a condition whereby a dog has such a strong mix of genes from both parents that it is larger, more disease resistant, and more fertile than either of its parents. However, evidence for hybrid vigour in dogs is limited, and emphasises the importance of treading carefully when deciding to mix two breeds.
The problem is that dogs have been selectively bred by humans for such a long period of time that they have developed a good selection of common genetic diseases and defects. Many of these conditions do not show unless the dog carries a full set of mutated genes associated with the disease. For example, degenerative myelopathy is a crippling and fatal disease that has no cure, and typically only shows itself when the dog is around eight years old. It is a recessive disorder, meaning that both alleles* of the gene must be defective in order to cause the disease. If a dog has only one copy of the defective allele, and the other is fully functional, then the dog will probably not show any sign of degenerative myelopathy. In fact, some dogs may have two copies of the defective gene and still not show signs of disease. Degenerative myelopathy is present in many breeds of dog, including Border collies and German shepherds. The gene controlling the disease is the same in all breeds. Therefore, if your dog carries degenerative myelopathy (you would not know unless you had it tested), and you bred it with a German shepherd that also carried degenerative myelopathy, approximately one in four puppies would develop the disease later in life.
Some dog breeds have genetic diseases that are unique to them, or rare in other breeds (such as dancing doberman disease). However, many dog diseases are prevalent in a range of different breeds, and controlled by the same genes. For these diseases, crossbreeding does nothing to prevent the puppies from developing the disease.
There are other problems, too, with crossbreeding to prevent disease. One study of 100,000 (mostly mixed breed) dogs found that mixed breed dogs were more likely to carry a recessive disease than purebred dogs. If you crossbreed, and either or both parents happen to carry a defective gene, then the disease is perpetuated. If those puppies have puppies, the gene is likely to be passed on, and will forever be a problem in that lineage. If, on the other hand, you breed from two adult dogs known not to carry a specific genetic mutation, then none of their puppies will have that mutation.
Does this mean that you should only ever buy purebred puppies? Of course not. Purebred dogs can inherit diseases just as easily as crossbreeds, and the large study mentioned above found that crossbred dogs were less likely to suffer from genetic diseases than purebreds. So for the lady with her Border collie in the dog park, and anyone else who is thinking of buying or producing dogs, the following take-home messages should be heeded:
1. If you buy a dog from a breeder, make sure that both parents have had all recommended breed-specific testing done, even if the puppies are not purebred. In doing so, you can feel smug that you have supported a responsible breeder.
2. If you breed from your dog, and you want to maximise the puppies' chances of living long and healthy lives, then make sure both parents are tested for genetic defects. (I need not mention that you must make sure that you are not adding puppies to a flooded market, and that your dog's line has no known potentially hereditary defects that won't be tested for, such as aggression).
Finally, if you adopt a dog, it will probably be healthy, especially if it is of mixed breed. However, keep in mind that it may carry genetic defects. Fear of genetic defects is not a good excuse for leaving a dog in the pound: any dog, purebred or crossbred, of known and unknown lineage, can develop issues (genetic or not) at any point in its life. Some health problems can be prevented or foreseen, but many cannot, and that is part of the burden we carry as pet owners. I would not swap my mixed breed rescue for any other dog in the world: if he has genetic defects, then whatever they are they are far outweighed by the strength of his character and the fun we have together.
*Some animals, such as humans and dogs, are diploid, meaning that they carry two sets of genetic information: one inherited from the mother, and one from the father. Each gene therefore has two copies, called alleles.
The idea that crossbreeds are healthier than purebred dogs is not new. In a way, the reasoning is sound: outbreeding introduces more genetic variation than inbreeding. Crossbred dogs can show hybrid vigour (heterosis), a condition whereby a dog has such a strong mix of genes from both parents that it is larger, more disease resistant, and more fertile than either of its parents. However, evidence for hybrid vigour in dogs is limited, and emphasises the importance of treading carefully when deciding to mix two breeds.
The problem is that dogs have been selectively bred by humans for such a long period of time that they have developed a good selection of common genetic diseases and defects. Many of these conditions do not show unless the dog carries a full set of mutated genes associated with the disease. For example, degenerative myelopathy is a crippling and fatal disease that has no cure, and typically only shows itself when the dog is around eight years old. It is a recessive disorder, meaning that both alleles* of the gene must be defective in order to cause the disease. If a dog has only one copy of the defective allele, and the other is fully functional, then the dog will probably not show any sign of degenerative myelopathy. In fact, some dogs may have two copies of the defective gene and still not show signs of disease. Degenerative myelopathy is present in many breeds of dog, including Border collies and German shepherds. The gene controlling the disease is the same in all breeds. Therefore, if your dog carries degenerative myelopathy (you would not know unless you had it tested), and you bred it with a German shepherd that also carried degenerative myelopathy, approximately one in four puppies would develop the disease later in life.
Some dog breeds have genetic diseases that are unique to them, or rare in other breeds (such as dancing doberman disease). However, many dog diseases are prevalent in a range of different breeds, and controlled by the same genes. For these diseases, crossbreeding does nothing to prevent the puppies from developing the disease.
There are other problems, too, with crossbreeding to prevent disease. One study of 100,000 (mostly mixed breed) dogs found that mixed breed dogs were more likely to carry a recessive disease than purebred dogs. If you crossbreed, and either or both parents happen to carry a defective gene, then the disease is perpetuated. If those puppies have puppies, the gene is likely to be passed on, and will forever be a problem in that lineage. If, on the other hand, you breed from two adult dogs known not to carry a specific genetic mutation, then none of their puppies will have that mutation.
Does this mean that you should only ever buy purebred puppies? Of course not. Purebred dogs can inherit diseases just as easily as crossbreeds, and the large study mentioned above found that crossbred dogs were less likely to suffer from genetic diseases than purebreds. So for the lady with her Border collie in the dog park, and anyone else who is thinking of buying or producing dogs, the following take-home messages should be heeded:
1. If you buy a dog from a breeder, make sure that both parents have had all recommended breed-specific testing done, even if the puppies are not purebred. In doing so, you can feel smug that you have supported a responsible breeder.
2. If you breed from your dog, and you want to maximise the puppies' chances of living long and healthy lives, then make sure both parents are tested for genetic defects. (I need not mention that you must make sure that you are not adding puppies to a flooded market, and that your dog's line has no known potentially hereditary defects that won't be tested for, such as aggression).
Finally, if you adopt a dog, it will probably be healthy, especially if it is of mixed breed. However, keep in mind that it may carry genetic defects. Fear of genetic defects is not a good excuse for leaving a dog in the pound: any dog, purebred or crossbred, of known and unknown lineage, can develop issues (genetic or not) at any point in its life. Some health problems can be prevented or foreseen, but many cannot, and that is part of the burden we carry as pet owners. I would not swap my mixed breed rescue for any other dog in the world: if he has genetic defects, then whatever they are they are far outweighed by the strength of his character and the fun we have together.
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| Besides, he's SO pretty. |
*Some animals, such as humans and dogs, are diploid, meaning that they carry two sets of genetic information: one inherited from the mother, and one from the father. Each gene therefore has two copies, called alleles.
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