Totally with junkyard on this, you get bad dogs like you get bad people.
The Chav dog of choice thing seems pretty regional and trends change, as a dog catcher that has a massive area i see quite often that in a certain area there will be lots of similar dogs of similar ages dumped/lost at similar times.
Chav dog of choice at the moment round our way seems to be Husky’s, these things make Staff’s look like a sensible pet as the can jump the average council house fence in one, hopefully American bulldogs never become in vogue, the Mastiff trend was a pita.