A beginners guide to inbreeding and line breeding

My thanks to Sue Bowling for allowing me to quote extensively from her article

 

First and foremost I should emphasise that line breeding is the cornerstone of selective breeding. Selective breeding has given us cows that give the maximum amount of milk, sheep that give the maximum amount of wool, chickens that lay eggs almost every day and the most beautiful dogs in the world. There is nothing wrong with line breeding but it is like using a satellite navigation device – if it is not used intelligently you land up in Richmond, North Yorkshire when you intended to go to Richmond in London! If you are to use line breeding intelligently you have to know the basics. I hope that this short article will help.

What are inbreeding and line breeding, and what effect do they have?

In genetic terminology, inbreeding is the mating of two animals who are related to each other. In its opposite, out crossing, the two parents are totally unrelated. Since all pure breeds of animal (including humans) trace back to a relatively limited number of foundation ancestors, all pure breeding is, by this definition, inbreeding though the term is not generally used to refer to matings where a common ancestor does not occur within a five-generation pedigree.

Breeders of purebred livestock have introduced the term ‘line breeding’, to cover the milder forms of inbreeding. Exactly what the difference is between line breeding and inbreeding tends to be defined differently for each species for there is no ‘formal’ definition. Inbreeding at its closest applies to what would be considered incest in human beings – parent to offspring or a mating between full siblings. However, uncle-niece, aunt-nephew, half sibling matings, and first cousin matings are called inbreeding by some people and line breeding by others. Under normal circumstances if this was the only example of close breeding in a five-generation pedigree what is called the ‘inbreeding coefficient’, expressed as a percentage, would be so low as not to be significant. But three things need to be taken into account. The closer this relationship is to the first generation of the pedigree, the more often it occurs and the relationships of the other sires and dams in the pedigree all result in an increased percentage.

What does inbreeding (in the genetic sense) do? Basically, it increases the probability that the two copies of any given gene will be identical and derived from the same ancestor. The higher the inbreeding coefficient the more likely this is to happen. The technical term is ‘homozygous’ for that gene. The ‘heterozygous’ animal has some differences in the two copies of the gene. Remember that each animal (or plant, for that matter) has two copies of any given gene (two alleles at each locus, if you want to get technical), one derived from the father and one from the mother. If the father and mother are related, there is a chance that the two genes in the offspring are both identical copies contributed by the common ancestor.

This is neither good nor bad in itself – but consider, for instance, the gene for PRA (Progressive Retinal Atrophy), which causes progressive blindness. Carriers have normal vision, but statistically, if one is mated to another carrier it is likely that one in four of the puppies will have PRA and go blind. Inbreeding will increase both the number of affected dogs (1 in 4) and the number of genetically normal dogs (3 in 4) so inbreeding can thus bring these undesirable recessive genes to the surface, where they could be removed from the breeding pool – you do not breed from the dogs which go blind – although a proportion of other dogs in the litter will be carriers. This will only matter if they are mated to another carrier, of course, but it demonstrates the complexity of the problems

Unfortunately, it is still much more complicated for we cannot breed animals based on a single gene – the genes come as just two packages: one in the sperm and one in the egg. So you may be able to eliminate one undesirable pair but the very fact that the animals will be becoming increasingly homozygous (which may quickly improve some characteristics) is also likely to bring other undesirable combinations to the surface.

Sewell Wright developed what is called the ‘inbreeding coefficient’ in the 1920s. This is related to the probability that both copies of any given gene are derived from the same ancestor. A total outcross (in dogs, probably a first-generation cross between two purebreds of different, unrelated breeds would be the best approximation) would have an inbreeding coefficient of 0. As we have seen, they would still have common ancestors many generation back so would still be homozygous for some genes shared by all dogs so even though the inbreeding coefficient = 0 even matings between unrelated pairs can still throw up genetic disease.

An inbreeding coefficient of 100% is rare in mammals and would result if the only matings practiced over many generations were between full brother and full sister. A mating between a brother and sister from unrelated parents would result in an inbreeding coefficient of 50%. A mother/son (or vice versa) or father/daughter (or vice versa) mating would result in a breeding coefficient of 25% assuming that there were no other related matings in the preceding generations. A cousin-to-cousin mating actually gives a relatively low percentage (6.25) but other related matings would affect this figure – perhaps substantially. However, Dr Malcolm Willis, one of the most experienced geneticists in the world of dogs, has said that the average inbreeding coefficient in pedigree dogs registered with the Kennel Club is actually only between 4 and 5% but, of course, the long term effect of many generations of a breed on the same register will mean that today’s dogs do have a higher chance of passing on deleterious genes simply because, as explained at the beginning of this article, there were relatively limited number of foundation ancestors.

As a general rule, very close inbreeding in domestic animals cannot be maintained for many generations because it generally results in loss of fertility – apart from any other genetic disease which may become apparent. .

To ensure genetic health breeders need to select pairs in such a way that the inbreeding coefficient of the offspring is kept as low as possible commensurate with the adherence of stock to the breed standard. One way of doing this is to use the method often adopted in other countries: that is breed from animals which ‘look’ the same (heterozygous) as distinct from what we tend to do in the UK which is to breed from animals which are genetically similar (homozygous).

Another key is to constantly move away from families known for possessing deleterious genes – a method practiced by knowledgeable dog breeders for generations.

You can download a programme from the Internet called GENEs which was written by Dr Robert Lacy which will enable you to calculate the inbreeding coefficient of any mating (assuming you have the full five generation pedigree) quite easily. The programme is free but has some restrictions. Go to http://www.vortex9.org/genes.html to download it.

Genetics is an immensely complex subject and this is but a simplistic introduction. Much more can be found at http://www.highflyer.supanet.com/coefficient.htm including the formula for working out inbreeding coefficients. It is:

 

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