<<There is no gene for "aggressiveness" - if there is, then humans would also have a gene for "aggressiveness." >>
But I'm sure that there are human genetic factors for aggression. Hostility. Some has to be environmental, sure, but I would think that natural selection would have (up to a point) favoured aggresiveness. Then in civilized times, it might have become a disadvantageous trait, but civilized time on an evolutionary scale must be very short compared to the uncivilized time when nature favoured aggression.
<<What you're seeing as aggressiveness is actually a dog's natural instinct for "pack" protection that is enhanced with training.>>
Well training's obviously important, but among well-trained dogs there must be some that turn on their masters more frequently than others do, and that difference - - if it exists - - could certainly be explained by hereditary factors.