My vote goes for genetics. The amount of genetic based topics is high yield. If you don't understand how genes play a role in disease states, the central dogma and its exceptions, etc, you will be lost bio/biochem sections. On the other hand, in the thousands of questions I've done from AAMC, Uworld, NS, etc., I've not encountered any physio that wasn't part of basic bio. Everything Ad2b mentioned I was tested over in bio II, so I guess it depends on what material your bio classes covered.