Ok, I wasn't going to post to this thread because it's been done several times already and whats been said was said. However, I must put in a word or two.. I didn't see this said, so I will, forgive me if it's already been reveiwed..
There are many visible differences between the breeds of domesticated dogs however, they are all in one species, Canis familiaris. This is the main reason why we call Dog types "breeds" and not species, so saying you have crossed a Rot and a Shepard isn't a valid argument, as for they are the same species.. Pure bred dogs have so many problems because they are insanely inbred and in fact are nothing but a mixture of multiple "Projects", and when they find a type they want they inbreed the animals and so forth and so on which happens to be another argument..