We feel very confident that it is genetic, but we don't know how it's passed. We know that it can skip generations. Here is the info I've gotten from the literature and our research: In Yorkies we see it when certain lines are bred together. We have breeders that have avoided breeding their lines to another specific line for 4-6 generations, because they got a shunt the first time they bred the two lines together. When they breed the 5-7 generation dogs back to that line, they get a shunt.
In Yorkies we know that it is not simple recessive, simple dominant or sex linked. We bred 2 shunt dogs together and got two healthy puppies. Based on our pedigree work and breeding trials, that leaves us with polygenic, incomplete penetrance, and variable expressitivity as the genetic pathway. That doesn't tell us about the parents, though, as far as whether one or both are affected.
If Cairn Terriers with micorvascular dysplasia (MVD or HMD) and no shunts are bred together, they will produce dogs with shunts and also dogs with MVD. This may support the idea of variable expressitivity - that dogs with mild forms of the disease only have MVD while dogs with severe forms have shunts. That means that the breeding dogs better have normal bile acids or they could be passing on shunt genes, even if the bile acid elevations are only mild (alternatively, several liver biopsies could be taken to prove the breeding dogs don't have HMD/MVD if the bile acids are mildly increased).
A pedigree study that is ongoing in Maltese has shown that affected dogs in the study group were all related to one of two(2) affected males.
The best pedigree studies are from the Netherlands on Irish wolfhounds-they identified 11 lines of IW in the Netherlands and five of these lines were more prone to having shunts. By not breeding affected lines to each other, they decreased the incidence of shunts from 50%.
Bottom line is we don't know how it's passed. All breeding dogs should have normal fasting and fed bile acids (true normal -- i.e. less than 10 and less than 20; versus "okay as a pet" normal, i.e. 25-40 range). At the very least, dogs that have produced puppies with shunts should not be bred to the same lines (their own or that of the other parent). Obviously the best breeders would never breed the parents again, and they may consider spaying and neutering the littermates as a "just in case" precaution. If, however, shunts are found in all lines of Yorkies, then even that may not solve the problem.
PORTOSYSTEM SHUNTS FAQ