mickrick

Because weak points get bred in aswell as strong points and the careful breeder is aware of this and knows what to do to elimenate the weakness.

The point about lines is that teriermen introduce differrent breeds into their stock in order to improve a certain point in the breed, i.e. introducing a staffordshire to improve strength, or a hound to improve nose, then breed worker with worker along that line. In any good line you should be able to find out what has gone into the breed and how far back it was introduced. Also it helps if somebody is keeping a record of what is being bred with what. Good hunting.