You make such an excellent point there jooly - as i am sure you could reel off a whole list of breed specific conditions that are a direct result of breeding dogs to a desired look. It makes me blood boil.
As far as i am aware, a Jack Russel is a term used for any small terrier cross breed, that resembles a jack russel - the variation is quite stark - some have short legs, some have quite strong jaws some don't etc etc. The Jack Russel is not a kennel club recognised breed and as such it would be difficult to be sure of lineage. The Parsons Jack Russel has recently been accepted by the KC, and they tend to be taller dogs with a more cutsey face as far as i can see, there would be more consistency within this breed.
I have a border terrier cross patterdale. Well, thats the theory. We bought him as such but all my friends, including three vets, tease me that he is a JRT. And when anyone sees him they ask me if he is a parsons jack. My friends say to me that the father couldn't of been a patterdale due to his colouring, he is black brown and white, typical JRT. We did see a picture of the purported father who was a lovely little black patterdale. The thing is, patterdales are meant to be a block colour although some can have white chest pups within a litter. My thinking is as follows: Not sure why the people we bought the dog from would lie, they were sold as cross breeds. I actually prefer my theory that the mother was not pure border terrier as she had a big white blaze, otherwise looked totally right for a border terrier. When we bought Bob i was a bit about the mothers credentials, but I bought him because, even though i wanted a border terrier, at the time we could only find dogs that were not bred in the home - Bob was home bred in a family with young children. The owners weren't professionals, i think they just fancied a litter of pups - they kept three of them!!! So my money although we did pay over the odds (£200) was well spent. So thats the story of bob the patterborder?dale!
One of my vet friends tells me that any small crossbred terrier is a JRT - but im a bit about that