What's the key to Jack Russell genetic diversity?
Simple: Not being in the Kennel Club. From "The challenges of pedigree dog health: approaches to combating inherited disease" at BioMedCentral:
In a recent UK study, several breeds were shown to maintain a high degree of genetic diversity, in particular the Jack Russell Terrier, which showed extensive admixture and very low levels of inbreeding. The Jack Russell is not recognized as a breed by the Kennel Club in the UK, where a similar breed, the Parson Russell Terrier is recognized. This provides UK Jack Russell breeders with a broader pool of potential mates for their animals, because it avoids the registration restrictions discussed above, maximising genetic diversity within the breed.