The fiction of 'knowing your lines'

bron: The Institute of Canine Biology
by Carol Beuchat PhD


Today I found yet another study that identifies a recessive mutation that has popped up out of nowhere to ruin the lives of some Labrador Retriever puppies with congenital myasthenic syndrome. It occurred in a pair of littermates from parents with two recent common ancestors. The investigators also examined relatives of these dogs and found 16 of 58 carried the mutation - that's almost 30% - while 288 unrelated Labradors carried the normal gene.

The authors say that "Linebreeding in this Labrador Retriever family makes it likely that the sire and dam inherited the mutation from a common ancestor and that the affected puppies are homozygous for a chromosome segment transmitted IBD" (identical by descent)... "Linebreeding practices expedite the appearance of recessive diseases in purebred populations."


Let me make this crystal clear.
This particular family of dogs carries a mutation that causes a serious genetic disorder, in this case the mechanism that allows nerves talk to muscles is broken in dogs that get two copies of the defective gene. Dogs with only one copy apparently are fine. Two related dogs were bred, both as it turns out were carriers, and they produced puppies with problems. However well this breeder might "know their lines", there was no way to know that this gene was lurking in this line of dogs.

Coefficient of inbreeding is the probability of inheriting two copies of the same gene from an ancestor on both sides of a pedigree. The higher the COI, the greater the risk of having something like this happen.  ALL dogs have mutations that the breeder has no way of knowing about. If this was a "responsible" breeder, they would have done the available DNA tests for the disorders known in Labradors. They could be certain of not producing puppies with any of those. But then they did a close breeding, and whoopsie, ran into this.

It's the mantra of the experienced breeder: "Know your lines." That is certainly good advice for the things that CAN be known, but there seems to be little appreciation for the fact that there are many things that you CANNOT know. The only way to manage the unknowns is by breeding in a way the manages the risk of finding out the hard way what those silent mutations are. This is what I argued in an earlier essay about why DNA testing is not going to solve the genetic problems in purebred dogs if breeders DNA test then inbreed. Just the other day I was reading a long discussion among breeders about inbreeding/linebreeding, and several breeders were swearing that they "knew their lines" and that's why they can linebreed without the genetic problems that other (less experienced?) breeders are complaining about. 

I don't know if these people truly believe the fiction that there are no problems in their lines, or perhaps the more serious myth that they can inbreed yet avoid genetic problems because of their great skill as breeders (and "knowing" those lines). What will it take to convince breeders that it's only a matter of time - the landmines are out there, and one of these days what appears to be a clear path is going to reveal them. Guaranteed.

I'm sick of reading these papers about yet another "new" genetic disease in dogs caused by a recessive mutation that has become a problem because of inbreeding/linebreeding. I'm sick of reading posts from breeders who proclaim that skill, experience, and "knowing my lines" allows them to breed closely related dogs without consequence, when in fact they are intimately involved in a dangerous game of roulette in which, sooner or later, the loser will be a puppy and the family with a broken heart that owns it.