Continued from page 2In order to otain a clearer idea of the effect of inbreeding on diseases of known or possible genetic origin the individual reports were plotted as a scatter diagram of lifespan vs inbreeding coefficient.
The green points are dogs that died from Addison's Disease and the light blue (cyan) are ones that died from various immune/autoimmune problems. I was somewhat surprised by the lower-than-average inbreeding of these individuals, but there are at least two possible explanations. One is that it is an artifact of small sample size (if I had more reports, the distribution would be more random). The second is that inherited problems severe enough to cause death at a young age are unlikely to be passed on in a line that is highly inbred.
Though the data is widely scattered, the points seem to fan out from a lifespan of 15-16 years at zero inbreeding (95% of the points are contained between the red and green lines). I interpret the data as telling me that the more highly inbred dog may still live 15 years, but his chances of doing so are reduced. If one looks at those dogs that died from cancer (pink), there is no apparent correlation between the incidence and inbreeding. This does not necessarily say that there is no genetic component, but suggests that any genetic predisposition to cancer is not affected by increasing homozygosity through inbreeding.
In contrast, those that died from bloat (red) tended to be more inbred than average. The percentage of deaths attributed to bloat increased with higher levels of inbreeding, while the average lifespan decreased.
I have suggested that the underlying defect which makes an animal bloat-prone may be inherited as a dominant trait (see Pedigree Analysis: Bloat in the Standard Poodle). Not all dogs carrying the gene are exposed to the non-genetic factors that trigger an attack, or may not suffer an attack until they are past breeding age. Coupled with the failure to recognize bloat as an inherited trait, I believe that this has resulted in heterozygous individuals being bred and homozygous pups being born. As most dominant traits are more severe when homozygous, this could well account for the shorter lifespan of the more highly inbred dogs.
The remaining, dark blue points, include dogs that died from all other causes, died of old age, or for which the cause was not reported.
In Summary
1. The potential lifespan of a SP is 15-16 years (the median age at death is actually 10.7 years).2. There is no evidence that the incidence of cancer, the most common cause of death, is affected by breeding.
3. There is an overall trend toward shorter lifespans with increased inbreeding and a particularly striking increase in the incidence of bloat.
4. The data also suggests that Addison's Disease, immune-mediated hemolytic anemia and some other immune/autoimmune problems have a genetic basis.
Additional notes:
- There were no obvious differences between males and females.
- There has also been no significant change in the average lifespan over the past 50 years. (However, I have been unable to establish the cause of death for a sufficient number of older reports to draw any additional conclusions.)
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I am grateful to all who have provided data, and to Elsa Sell, Health Committee Chairperson, Bearded Collie Club of America, for allowing me to use her mortality data.
References:
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© John B. Armstrong, 1998.
Revised August 26, 1998
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