I can't speak for Kennel Club purebreds, but I know the Jack Russell Association requires that there be at least 5 generations apart in any possible relationship to each animal.
As they don't want the inbreeding issues that other breeds have. And they want no part of the Kennel Club either, that's why they formed their own association.

Which is probably why Jack Russells ( or Parson Russells) continue to be one of the most intelligent dogs out there( we won't discuss the rebelliousness of them though )



"A" is for Atonal, artfully adding audible auras