2007 AKC CHF ANNUAL CONFERENCE
RESPONSIBLE BREEDING MANAGEMENT OF GENETIC DISEASE
Jerold Bell, Tufts
To clear up a misconception: Mixed breed dogs are NOT inherently free of genetic diseases. Although that may be the case for rare, breed-related disorders, the common diseases are seen at the same frequency in both mixes and purebreds. For example, hip dysplasia is seen at the same frequency in pure and mixed breeds. Inherited hypothyroidism is seen in 7.5% of purebreds, and 10.7% of mixes (but not many of the purebreds are tested as a screening tool for breeding, so this is not a fair comparison). Nine of the ten most common inherited disorders for all pure breeds are seen just as commonly in mixes.
One reason for this is that some disease causing genes mutated so long ago that they were present in founding breeds from which other breeds, and mixes, diverged. One such example is the prcd form of PRA, found in at least 17 diverse breeds. In such cases, it's not so much that these affected breeds suffer from founder effects, but that unaffected breeds lucked out on founder effect or accidentally lost the defective gene somewhere along the line. Mixes between breeds sharing disorders, share the same disorders. Cockapoos and Labradoodles both have prcd-PRA because Cockers, Labs, and Poodles all have it. Labradoodles also are seen with hip dysplasia, elbow dysplasia, hereditary Mixes b's, just like both parent breeds.
So it's a question of what is a healthy breed so much as what is a breeder who tries to produce healthy dogs. And that breeder is one who makes use of the genetic testing available for that breed, and makes those results available to the public. For more details on how to deal with disorders with different modes on inheritance, go to http://www.vin.com/tufts/2007.
The popular sire effect is the #1 cause of genetic problems in pure breeds. Not only are the popular sire's genes overrepresented in the gene pool, but his use pushes aside the use of other quality males that could be contributing, removing those dogs' genes. Managing genetic disease depends on what kind of test you have. If you have a direct genetic test of a linkage test you can make strong assumptions about carriers. If you can detect carriers, then your strategy should be to breed carriers to normals, then replace the carrier with a normal offspring for breeding in the next generation.
If you have no genetic test, then you need to know the status of relatives, both ancestors and siblings. Without any tests available, breed high risk dogs to low risk ones, replacing high risk dogs in the next generation. To do this requires an open health database. But without genetic tests, the effect of selection on the gene pool is minimal. If everyone decides not to breed to carriers, it can have a significant limiting effect on the gene pool---this is the worst decision you can make for breeding.
An individual is not a hip, eye, or heart---making breeding decisions based on a single test is inappropriate.
To clear up a misconception: Mixed breed dogs are NOT inherently free of genetic diseases. Although that may be the case for rare, breed-related disorders, the common diseases are seen at the same frequency in both mixes and purebreds. For example, hip dysplasia is seen at the same frequency in pure and mixed breeds. Inherited hypothyroidism is seen in 7.5% of purebreds, and 10.7% of mixes (but not many of the purebreds are tested as a screening tool for breeding, so this is not a fair comparison). Nine of the ten most common inherited disorders for all pure breeds are seen just as commonly in mixes.
One reason for this is that some disease causing genes mutated so long ago that they were present in founding breeds from which other breeds, and mixes, diverged. One such example is the prcd form of PRA, found in at least 17 diverse breeds. In such cases, it's not so much that these affected breeds suffer from founder effects, but that unaffected breeds lucked out on founder effect or accidentally lost the defective gene somewhere along the line. Mixes between breeds sharing disorders, share the same disorders. Cockapoos and Labradoodles both have prcd-PRA because Cockers, Labs, and Poodles all have it. Labradoodles also are seen with hip dysplasia, elbow dysplasia, hereditary Mixes b's, just like both parent breeds.
So it's a question of what is a healthy breed so much as what is a breeder who tries to produce healthy dogs. And that breeder is one who makes use of the genetic testing available for that breed, and makes those results available to the public. For more details on how to deal with disorders with different modes on inheritance, go to http://www.vin.com/tufts/2007.
The popular sire effect is the #1 cause of genetic problems in pure breeds. Not only are the popular sire's genes overrepresented in the gene pool, but his use pushes aside the use of other quality males that could be contributing, removing those dogs' genes. Managing genetic disease depends on what kind of test you have. If you have a direct genetic test of a linkage test you can make strong assumptions about carriers. If you can detect carriers, then your strategy should be to breed carriers to normals, then replace the carrier with a normal offspring for breeding in the next generation.
If you have no genetic test, then you need to know the status of relatives, both ancestors and siblings. Without any tests available, breed high risk dogs to low risk ones, replacing high risk dogs in the next generation. To do this requires an open health database. But without genetic tests, the effect of selection on the gene pool is minimal. If everyone decides not to breed to carriers, it can have a significant limiting effect on the gene pool---this is the worst decision you can make for breeding.
An individual is not a hip, eye, or heart---making breeding decisions based on a single test is inappropriate.