No pictures today.
The puppies are the result of a test breeding for an inherited eye defect – retinal dysplasia. A recessive genetic defect, the dysplasia can be severe enough to cause blindness through retinal detachments and is present at birth.
I had the misfortune to discover this gene had been quietly introduced into a previously clear line I had been carefully building for nearly 20 years. Because it was recessive, there were no clues as to what was happening. None of my original dogs carried the gene, so the puppies continued to be born with normal eyes, though more and more were genetic carriers. By the time the first affected was born, the damage was done. Most of my top breeding dogs were carriers, or offspring of carriers, including two who have nearly 30 Best In Show wins between them.
The only silver lining is that RD is present at birth, and that makes it controllable. We can ensure that no buyer is ever sold an affected puppy through a routine opthalmologists examination.
I agreed to participate in a formal research program at the veterinary college at the University of Saskatchewan to research the defect (it was previously unrecognized as a breed related defect) and to establish the mode of inheritance. Representational differencial analysis is being used to try to locate the actual gene responsible and develop a screening test.
But that doesn’t solve the immediate problem. Which dogs to breed? Carriers are phenotypically identical to clears. Most animals carry an average of 5 defective alleles (humans included) , and purebred dogs generally carry even heavier genetic loads due to their small founder base and closed gene pools. Simply discarding high risk dogs and starting over wasn’t a rational solution. The line was very healthy in other respects, and clear of some nasty health problems that are common in other families.
The goal became to eliminate the gene itself, over the span of several generations, preserving the quality and general good health of the animals. Short form – dump the gene, save the line. Or, failing that, save the line and control the gene by planning breedings that prevent its expression.
The actual sifting process involves test matings of normal eyed dogs to mates who are homozygous affected. (The affected test dogs live normal lives as house pets, and most are sighted.) A simple, autosomal recessive trait, retinal dysplasia “takes two to tango” . The puppy must recieve two defective alleles, one from each parent, to have the defect.
If so much as a single puppy is affected, it proves the normal eyed dog is a carrier and any breeding must be carefully controlled, if he/she is bred at all.
If the puppies are all normal eyed, a mathematical probability is assigned to the dog tested. A dog with 3 normal puppies from an affected mate has a 87.5% probability of being clear of the gene – a dog with 6 puppies has a 98.5% probability clear. By using dogs with a “high probability clear”, in favour of unknowns and retiring carriers, we prevent the defect is being spread into the gene pool.
Recessive defects are particularly damaging in a closed gene pool where it is possible for one dog to sire hundreds of puppies in a relatively short time. One carrier sire can turn a rare defect into a very common one in only a few generations, especially if a few of his sons become similarly popular at stud. So, while crude, and unpleasant, a litter of affected puppies can prevent the birth of many, many more, in generations to come.
At least, that’s what we tell ourselves.
The puppies were examined today, and of the seven, five were affected with retinal dysplasia. The laws of probabilities are not kind. A large litter can be a train wreck. With a guarded prognosis and little demand for test breeding males, I left two of the male puppies behind. Their retinas were harvested to provide additional RNA for dna studies. Small dead animals.
The two who are normal will go to pet homes, and will be sterilized. The affected male puppy is going to Maryland to be used to test breed several females in kennels in the US, one of the affected females is going to a loving pet home that has had previous experience with a blind dog. The third affected female’s fate is uncertain at the moment. She may have a future as a test breeding bitch, she may not.
Me? Me thinks I need a another drink.
