To understand test breeding and why it is sometimes a useful tool for the control of genetic diseases in purebred dogs, a brief explanation of the genetics that underlies test breeding is in order.Most of us learned about the pea breeding experiments of Gregor Mendel in our grade school science classes. His experiments demonstrated how it is possible for a gene to remain hidden for generation after generation only to pop up when it meets another gene of its kind in the lucky (or unlucky) offspring. Such genes are called "recessives". One easily understood example of a recessive gene is the one that creates blue eyes in humans. The gene for brown eyes is dominant, and if one parent has brown eyes, there is no way to know if they carry the gene for blue eyes, unless they marry a blue eyed person or a brown eyed person who also carries - and blue eyed children are born.
Recessive genes can also be responsible for defects. When we encounter a defect in our purebred dogs, one of the first questions we ask is "how is this inherited?". To control the spread of defects, we must gather as much information as possible. Because normal dogs can carry them, it is not enough to simply not breed from the dogs with problems - it does little to eliminate the defect from the breed. We have to find tools that identify which of those normal dogs are carrying the hidden gene.
Using pedigree analysis, testing and research breedings, it is sometimes possible to establish that a defect is caused by a gene that is recessive - and when this is the case, a protocol called "test breeding" can be very useful to help clear a breed of damaging genetic defects.
All traits are controlled by gene pairs. If the normal dog is is free of the defective recessive gene, he cannot produce the defect, no matter the status of his mate. He is clear of the gene. However, it is possible for a normal dog to have one copy of the dominant normal gene, paired with a recessive, hidden defective one.
"Test Breeding" is a protocol that is used that tries to force this hidden gene to the surface, so that dogs who carry it can be identified and removed from the gene pool. The usual way this is done is to breed the normal dog to one who is affected with the defect. If any of the puppies from this pairing are affected, we will have determined that the normal eyed dog is a "carrier" and remove him from breeding. If all the puppies are examined and found to be normal, we can assign a mathematical probability to that dog being clear of the gene. The more normal puppies there are, the higher the probability is that he does not carry the defect. Those probabilities can then be used by breeders to steer them towards dogs who are safer to use.
Test breeding does have its limitations. It is impractical for defects that take years to show symptoms and inhumane if the defect being studied causes great pain or extreme disability. But for some disorders, it is a practical and effective tool for improving the health of gene pools.
One disorder brought under control using test breeding in Miniature Schnauzers was a defect known as Congental Cataracts. Identified in the 1970's, it was once very, very common. Test breeding helped to virtually eliminate it from show stock over the next 15 years, and today it is rare for any puppy to be born with these blinding opacities.
Retinal Dysplasia is also a disorder that we believe may be controlled by test breeding, as it appears at this time to be recessive in inheritance. It is a painless disorder, and many affected dogs still retain much of their vision. They suffer no other known health defects in association with the gene, and can be diagnosed at a very young age. The defect is one that occurs during fetal development - it is not a degenerative disease. Thus, test animals have a good quality of life. Many can live normal lives as pets, if their dysplasia is mild or moderate and they are still sighted.
So what happens to the test puppies? Well, when the test mating has been done, puppies are raised like any other normal litter. At about 8 weeks of age, they are examined by a certified Veterinary Opthamologist for signs of the defect (and any others). If they receive a clean bill of health at this time, there is no further danger that genetic retinal dysplasia will occur at a later date - just as a brown eyed child will never develop blue eyes! Any puppies who have the defect will be assessed on the severity of the condition. Mildly affected dogs can also live perfectly normal lives as pets, as the condition is not thought to be progressive. And some affected animals will go on to be valuable test breeding dogs for the next generation, or for other breeders who find their dogs at risk.
In short, a normal eyed test puppy is a normal puppy. He has no more or less risk of developing other eye or health defects than any other well bred Miniature Schnauzer. He carries a recessive gene for the defect, and is capable of producing more carriers or affected puppies if bred - thus, spaying and neutering of test puppies is compulsory.
If they are normal, then why is a test puppy a little less expensive? Because we know he is going to a pet home from the day he is born. Some of the time and expense associated with growing out a litter for show purposes is not required, so this is reflected in the price. Also, as a breeder, I feel that your acceptance of such a puppy as your family member is worthy of appreciation, and I try to reflect that in adjusting the price somewhat. If you choose a mildly affected puppy as your pet, considerable accomodation in price is a given. However, all puppies, whether normal or mildly affected, are covered by the same comprehensive health guarantee, are registered, and recieve the same vaccinations and quality of care and socialization as any Minuteman puppy. And all puppies will have as a part of their health records, a copy of their official eye examination form from an AVCO certified Veterinary Opthamologist.
Catherine McMillan
Minuteman Perm.Reg.
Information on Retinal Dysplasia from Purdue University