Eggleston (2000) reported on the range of genetic diversity among the AKC breeds. She constructed a continuum for all of the breeds. At one extreme she placed the Bull Terriers which had the least amount of genetic diversity. This means that they tend to be line or inbred. At the other extreme were the Jack Russell Terriers who she found to have the most amount of genetic diversity. This means their pedigrees were for the most part the result of outcross breedings. This meant that the ancestors tended to be unrelated to each other. :: source
Not said about the Bull Terrier: the reason the Bull Terrier is the MOST inbred dog in the American Kennel Club today is that this breed started out as a "dog dealers dog" created in England wholecloth by one James Hinks. The initial pool of Bull Terriers admitted to the British Kennel Club was very small, and this gene pool was further reduced and bottlenecked by sire selection, and the splitting of the breed by coat color and size.
Not said about the Jack Russell Terrier: The reason the Jack Russell Terrier is the LEAST inbred dog is that the dog was only recently admitted to the Kennel Club, and almost all the dogs that entered the Kennel Club Registry were dogs whose pedigree originated in the Jack Russell Terrier Club of America, which prohibits a Coefficient of Inbreeding higher than 16% and which also maintains a wider (true working) standard for the breed. The American Kennel Club now calls its dog the "Parson Russell Terrier," while the JRTCA (quite correctly) notes that their dog is "The Real Jack Russell" actually found in the field and at work.