Hip dysplasia, I believe is what you might be refering to. Frightening....
I have always really liked very large dogs. Both our Bullmastiffs were pet quality, pure breed by highly regarded (By Bullmastiff society) recommended local breeders. I agree 100% in doing your homework. Unfortunately both our dogs (Thor and The Toecutter (ever seen MadMax?)) died of bone cancer when they were right around 10 years old (each came from a different breeder). They were very gentle with toddlers and all sizes of kids. Didn't have a mean bone in their bodies. At their size, they didn't need to be mean and badass. Very nice deep bark. Just a single, deep
WOOF and I've had salesmen tell me nevermind as I opened the door and they were headed on their way quickly back down the driveway.
On the other hand, pound for pound and ounce for ounce the Jack Russell is pure hell on wheels and carry their balls one behind the other for speed. Just wish the Jack Russell Terrier came in the 50lb size. Would be next of kin to the Tazmanian Devil. All balls but just not very smart. I've seen ours stomp, charge and bark at a new trash can placed in the kitchen. Like I said, pure hell on wheels just dumber than dirt
