I haven't seen such a breed before, but I do know that dobermans are notorious for their dominate genetic traits, so if the dog is a 50/50 split or less (mastiff) then it's likely the dog will look very similar to a doberman. The most likely difference would be a broader chest and/or face then the average doberman, around 10-15 lbs more weight, and possibly some fur discolor (lighter colored fur which may present as a vertical stripe on the front of the chest).
