Standard Poodle bloat pedigree. Circles indicate females, squares indicate males. Solid symbols indicate dogs that have bloated. Except for the litter of 7 (lower right), the status of the sibs of other individuals in the pedigree is not known.
Analysis of the pedigree
When attempting to establish the mode of inheritance from a pedigree, the objective should be to come up with the simplest model that will explain the data. Therefore, we should first ask whether bloat can be explained as a single-gene dominant or recessive trait.
Recessive trait (AA and Aa normal, aa bloats)
Expectation:
- two affected individuals should produce only affected progeny
- one affected and one normal should produce 1/2 affected and 1/2 normal progeny if the normal is a carrier (Aa)
- two normal may produce 1/4 affected progeny if both are carriers
Data:
- We have two examples (A and D) of affected individuals produced by normal parents, but lack data on sibs and cannot establish whether the proportion is approx. 1/4.
- The litter of seven has 3 affected - acceptably close to the expected 1/2.
- C (unaffected) is the daughter of two affected individuals. To account for this, we would have to say that that the trait is not fully penetrant (an individual genetically predisposed to bloat does not necessarily do so).
We also have to suppose that every non-bloating individual that has an affected son or daughter is a carrier. There are nine of them, only two of which are known to have a parent that bloated.
Dominant trait
(AA and Aa affected, aa normal.)Expectation:
- two affected individuals may produce unaffected progeny if both are heterozygous (Aa)
- one affected and one normal should produce 1/2 affected and 1/2 normal progeny if the affected is heterozygous
- two normal individuals should not produce any affected progeny
Data:
- C (unaffected) is the daughter of two affected individuals. As we lack data on sibs, we cannot establish whether this occurs about 25% of the time.
- The litter of seven has 3 affected - acceptably close to the expected 1/2.
- Affected individuals A and D are from two normal parents. To account for this, we would have to say that that the trait is not fully penetrant (an individual genetically predisposed to bloat does not necessarily do so) and that at least one of the parents is heterozygous.
However, in this case no restrictions are placed on the "unrelated" non-bloating parents of dogs that bloated.
Conclusion
Both the dominant and recessive model require us to propose that dogs with a "bloat" genotype do not necessarily bloat. In light of the Purdue study which indicates that stress, diet and other environmental factors play a role, this requirement is not unreasonable. What a dog inherits is a predisposition to bloat, not bloat itself.
As we can get a bloat genotype among the progeny of a normal and an affected parent, we cannot use this data to distinguish between the two possibilities. However, the recessive model requires that all these normal dogs be heterozygous carriers, whereas the dominant model places no restrictions. Therefore, on balance, I favor a dominant model.
Comments are invited. Email to Dr. John Armstrong, jbarm@uottawa.ca
© John B. Armstrong, 1997
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