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Effects of Inbreeding - Expectations
A second objective was to evaluate the impact of inbreeding on lifespan and the incidence of genetic problems. Among dogs that are only very slightly inbred, I would expect to find that some die from genetic and others from non-genetic problems. The frequency with which the former appear will depend on how common the "bad gene" is in the population. The frequency will be increased by inbreeding, because inbreeding increases homozygosity. However, if the problem leads to early mortality, continued inbreeding should tend to eliminate it. One might therefore predict that the highly inbred dog could be substantially free of genetic diseases. In contrast, nongenetic problems should be largely unaffected by inbreeding.However, though I agree that inbreeding can be used to identify undesirable mutant genes, there are several complicating factors. First, a genetic problem may be dismissed as nongenetic, particularly if it is not fully penetrant. Second, a dog may be bred, and it's progeny bred, before a late-onset problem is evident. Unfortunately, even a well-publicized announcement by the owner may not discourage the use of the descendants. These factors may actually increase the incidence of the problem in a highly inbred line.
As an added complication, inbred lines may accumulate sub-lethal alleles that, individually, have no particularly obvious effect (and are not selected against) but collectively reduce overall fitness. The relative frequencies of the different primary causes of death may or may not change as a result.
Effects of Inbreeding - Results
When we break down the results into 4 subgroups according to the level of inbreeding (based on a 10-generation pedigree that is at least 95% complete), the survival of those inbred to less than 6.25% (the equivalent of first cousins who shared no other common ancestry) is significantly greater than for the more highly inbred dogs.
Standard Poodle survivorship at different levels of inbreeding. Blue diamonds: < 6.25% (N=39); pink squares: 6.25%-12.5% (N=65); red triangles: 12.5-25% (N=141); black circles: > 25% N=71). The solid line is fitted to the > 25% group. 
The least inbred group survive, on average, 14 years -- approximately 4 years longer than the most highly inbred. Their survivorship more closely resembles that of a non-inbred human population in Europe or North America.
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