and this is the problem with cross breeding... you got a big dog (mastiff) and you want to put more drive so you add an apbt in theory you come out with a smaller than mastiff but bigger than apbt with more muscle and harder drive...right? well just like communism and socialism it only works well in theory alone. no one can predict how the genes will come together, how right or wrong it will be. Yes, there might be one, 2 or even the whole litter that comes out the way you wanted but more than likely you will end up with something that you did not was in size, temp, muscle/skeletal structure, etc. this is why pure breds are bred. they have consistently (with few exceptions) produced desired traits for the function that someone wants...want a fast dog? get a greyhound, want a ratter? get a terrier, etc. there are breeds of dogs with temp, size , structure, etc for pretty much what we want. Now if one finds him/herself in need of a dog with a particular function that has not been bred then by all means try to create one but do so in a responsible manner and have the means to see it through.