Importance is an interesting concept. Importance with respect to what?
Importance for preparing for the rigors of medical school? Anatomy and Physiology are equal in importance in that regard, although many find gross anatomy extremely difficult.
Importance for MCAT? There is an extremely small amt of anatomy on the MCAT. Physiology is a tremendous help.
My vote, seeing as you haven't taken the MCAT yet, would be for you to take physiology. I wish I had. I would have at least gotten a 10 or 11 on Bio 🙁 Had to settle for a 9... wish I had learned O2 dissociation curves and EKG/ECG readouts... would have helped for MCAT passages.
Either way, I recommend a prep course... others will disagree. Kaplan is responsible for my 21 to 32 increase, January 06 diagnostic to April 06 MCAT.
Genetics has very little importance for the MCAT. I just took 400 level genetics, and I don't think that any of that was important for the MCAT with perhaps the exception of some of the immunology stuff we touched upon just before the final... right now, you have to weigh everything on a cost-to-benefit analysis. One course grade isn't as important as your MCAT performance, IMHO.