Out of the three choices, I agree that Genetics is the way to go. Is it absolutely necessary? NO. Can you learn MCAT genetics without a dedicated course on the subject? OF COURSE. Will a genetics course make MCAT genetics questions child's play? PRETTY MUCH. Genetics makes up a lot of the questions in Bio Sci on the MCAT.
Biochem, Genetics, and Anatomy and Physiology were the most helpful courses for me for the MCAT. It's the physiology that's helpful--not so much the anatomy. Also, most people get the phys from Intro Bio II, but I didn't pay much attention in that class.