Completely agree with this. Retakes will do you well for DO schools. Taking upper-level bio classes will make the C in Gen Bio seem rather insignificant. For example, I made a C+ in Genetics, but I took an advanced-level Human Genetics and made an A. This isn't grade replacement, but I showed that I can understand the concept at an advanced level that shows I went through a rough patch in college (outlined in PS/secondaries.)
This, however, only works if the lab and lecture are graded as one entire class. My gen chem/organic chem labs were graded separately from lecture (had two separate grades.) However, some upper level bio classes, as well as physics classes, integrated the lab and lecture into one grade. So yes, doing well in lab brought up a few grades in the latter classes, but wouldn't do me anything to bring up my lecture grades in the former classes.