Understanding a Dog's Breeding Coefficient
Inbreeding to preserve the purity of a bloodline is as old as time. Cleopatra was married to her brother. The Habsburg dynasty had been intermarrying for so long that one of Charles I's, ancestors, Joanna of Castille, appears in his family tree 14 different times. In fact, Charles I was more inbred than he would have been if his parents had been brother and sister.
And so it is with purebreed dogs and one of the most common criticisms of pedigree dogs in particular. They are bred from a limited gene pool and all genetic problems that come with restricting a breed's diversity, loss in vitality and vigor, lower, smaller offspring, earlier mortality, shorter lifespans and the higher the level of inbreeding, the greater the detrimental effects
The problem is, that if you want want your sheepdogs breed with a strong instinct for herding, or your retrievers to keep retrieving, you need to strengthen those traits by, we, inbreeding.
And inbreeding (or Line Breeding) as some breeders prefer to call it, does exaclty that. It cements those trais of a breed you wish to bring through to the next generation, and all the crap genes with it.
Science helps us mitigate the dangers of inbreeding, however, by helping us calculate the statistical risk called the Coefficient of Inbreeding.
The COI estimates the level of inbreeding that results from a particular cross so we have a quantitative way of evaluating both the risks and benefits.
The coefficient of inbreeding is the probability of inheriting two copies of the same allele from an ancestor that occurs on both sides of the pedigree.
These alleles are "identical by descent". The COI is also the fraction of all of the genes of an animal that are homozygous (two copies of the same allele). So, for a mating that would result in offspring with an inbreeding coefficient of 10%, there is a one in 10 chance that any particular locus would have two copies of the same allele, and 10% of all of the genes in an animal will be homozygous.