Most of the time, if you didn't have the grades you don't get the interview. Once you finish the interview, they score your application+interview as a whole.
So yes, you can get rejected based on application or interview alone.
Btw, fastest way to find yourself in the reject pile after a interview, is to screw up the ethical portion of a interview. Usually ethical questions make up one or two questions of a interview, and if you get it wrong, the rest of your application won't save you from the reject/alt/waitlist pile.