Oz Catz - Australia's Ocicat Breeder - Inbreeding or Line Breeding

Answer: Ah, the old line-breeding question. I spoke on this at the recent NAVHDA annual meeting. I’ll give you the relevant genetics foundation, and then my opinions, and let you decide what to do.

Based on the number and frequency of known recessive lethal alleles (genes), it is estimated that each of us is walking around with 10 to 20 lethal alleles (but I know very few people who suspect they are carriers based on family history). Since dogs [and cats] have a similar numbers of genes, there is no reason to believe the situation is any different in dogs. Being recessive, these alleles cause no problem unless an individual is homozygous (has two copies - got one copy from each parent). For any one of these lethal alleles (and every other allele), the likelihood that an offspring carries that allele is .5. Think about it - if an individual has any particular allele, where would we look to find another copy? In a close relative, of course.

So suppose we mate a mother and son - from a probability (Pr) approach, suppose the mother carries lethal allele z, then the chance that the son also has z is .5, and the chance that a progeny from the mating gets two copies of the allele is .5 (Pr of getting it from mother) x .5 (Pr that father got it from his mother) x .5 (Pr father passes it to progeny if he has it) = .125. That says for any one puppy, there is a 1 in 8 chance it will be affected (i.e. die); or (same logic) 1 in 8 puppies will inherit the lethal condition - this may be reflected in reduced litter size if the lethal acts during gestation.

Now, we need to consider the situation across the multiple lethal alleles being carried: the chance of a puppy not being affected by z is .875 (that is, 1 - .125), probability for a puppy not being affected by either of 2 lethal alleles is (.875)(.875) = .766, for 3 lethal alleles is (.875)(.875)(.875) = .67; 4 alleles = .59; 5 alleles = .51, etc. So you can see, even if the mother carried only six lethal alleles (below the expected average), the chance that a pup produced from the mother/son mating is not homozygous for one (or more) lethal alleles is less than .5. Then if we start to consider all the recessive undesirable alleles that are not lethal, it is easy to see why close inbreeding has such devastating results.

Research from livestock/poultry/plant species is virtually universal in showing negative effects of inbreeding, even inbreeding at low levels. The traits most affected by inbreeding are the fitness traits, e.g. survival, health, disease/parasite susceptibility, reproduction, longevity. I am currently working with two Ph.D. students on oyster breeding projects at the OSU Marine Science Center - close mating in oysters virtually eliminates the stocks despite each female producing literally millions of eggs.

As a geneticist and a GSP breeder, it is my personal view that it is irresponsible for a breeder to sell (or even give away) dogs that are the result of close matings. Quite apart from the losses incurred by the breeder from the lethal situation, the real negative impact is the reduction in health and longevity that would be expected in inbred puppies - a cost borne by the owners of those puppies. Four years ago I made a deposit for a puppy to be produced from a particular mating - due to changing circumstances, the breeder was considering changing the mating arrangement to mate the bitch to her sire (much as your situation) and asked my opinion. I responded that I had no reservations about paying the $1,000 for the original intended pup, but that if the mating were to become father/daughter, I would not take the pick of the litter if she gave it to me free! Why would I want to invest years of attention and training into a dog with expected increased health and reproductive problems and likely to have a shortened lifespan?

One of my favorite genetics quotes puts line-breeding into perspective: ‘If one produces a litter of puppies from a mating of closely related parents, the good ones are line-bred, the poor ones are inbred’. So why do we see a number of good dogs that are the result of line-breeding? Probably because they are the select few good ones of the multitudes that are produced - inbreeding does not preclude producing a good dog, it just greatly reduces the probability for any one dog.

Poultry breeders use inbreeding extensively to produce uniformity in their product. Of course, if they begin with 100 lines and lose 95% of them during the inbreeding process they can still achieve their goal. The classic ‘success’ story of line-breeding in dogs is the Elhew line of Pointers - its founder, Bob Wehle, made numerous matings of closely related individuals, but then he ruthlessly culled dogs, often entire litters, that were disappointing. After reading his book ‘The Making of Snakefoot’, I concluded that his breeding program was successful in spite of inbreeding, not because of it. Intense selection was the foundation of his success.

That’s my 2 (10?) cents worth. Just as an individual may have driven drunk dozens of times without killing anyone, individual exceptions don’t negate average expected outcomes. Good luck in whatever decision you make.

In-Breeding or Line-Breeding