With mixed breeds, appearance can vary so much-- a couple months ago a brindle Pit-bull-type mother named Myra, gave birth two a litter of what looked like two yellow labs, one mix, and one brindle pit-mix like her:
(Below are Myra and one of her puppies, Ring Ding.)
In certain jurisdictions, Breed Specific Legislation mandates that dogs be assessed based on physical attributes, not behavioral testing. In Denver for example, whether or not a dog is a pit bull depends on an assessment based on 36 points- measuring the head size, chest, paws, etc. If a dog is found to be a pit bull, you have 24 hours to remove it from the city.