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.
I did an analysis of why the bull terrier got inbred. I actually wanted one at one point, and I became very well-versed in its pedigrees and history (not as much as I am with retrievers).
You can read the analysis here, if you're interested: http://retrieverman.wordpress.com/2009/01/12/why-are-bull-terriers-so-inbred/
And it is exactly as you say. This breed went through all sorts of changes. When UK banned ear cropping, they had to inbreed to get the erect ear. Actually, the first modern bull terrier wasn't born in 1917. From him, all modern dogs descend.
But what amazes me is the denial. I was accused of not knowing the breed history on a forum, when all I used as source were their breed club's official histories!
These people don't understand historiography at all.
Or those people don't want to know the sad history. Unless we find a good way to out breed all these pure bred dog types it's all going to end tragically.
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