Hey Playah,
I did excellent on all the basic science sections and my advice is still don't waste your time. 90% of the exam focuses of path, pharm, and micro. Try to get more clinical knowledge since that is the most helpful.
Your studying should be geared toward a combo of what the test emphasizes and what your weaknesses are.
My studying went like this: 1/2 day of epid, genetics, histo, and neuro. 1 day of biochem and anatomy. 3 days of physio. 3 days of pharm, 3 of micro, 4 of path, and multiple full day practice exams and time spent reviewing my answers. Total, I spent about 3 1/2 weeks studying before taking the test. This is of course in addition to working my tail of during my first 2 years and trying to learn as much as possible for my classes (the best way to a good Step 1 score).
My downfall was pharm. It felt like a 3rd of the test was pharm but that's only because it was my worst subject and I noticed how hard those questiosn were. Like I did, most people empahsize their worst areas, for many that is probably genetics and biochem. Furthermore, while there may be an increasing emphasis on gentics, biochem, and molecular biology, don't interpret that to mean that these are anywhere close to a huge portion of Step 1.
If I were to change my study schedule I would have eliminated histo and anatomy, since histo isn't tested other than relevant path, and anatomy I knew like the back of my hand, and focused that time on pharm since that is where I sucked.
The test seemed to have the following proportions for me: 65% path (half of these questions ultimately were biochem, pharm, micro, or genetics, but the stem was usually diagnosing the disease or something else you learned in path). In my opinion the BRS path is the best resource. 10% entirely pharm. 10% entirely micro. 15% for basic science stuff, epi was probably the heaviest on my test. I know I only had 3 anatomy Q's, and my only neuro Q's were the last 3 on my test (Wallenberg syndrome, stroke, and MS). Genetics, Biochem, and Molecular bio were certainly there, and while none where strengthes in med school (granted I did get A's in all) I wouldn't say I remember any, or that they were a significant portion of the test. Bottom line, know your path, and if you don't, start learning it now.
If you still want an answer to your question about the best genetics book, I would say don't use anything other than High Yield unless you made extensive highlights/notes in the longer review books or make superbly conise notes for each class. I hated High Yield while going through my classes because they just weren't thorough enough, but for Step 1 all of your classes over teach you. You need to refresh the basics of every subject super fast (you have 1/2 to 1 day for most basic science stuff), and High Yield is the best way to do this.
Remember often that you love learning (or you wouldn't be here), otherwise it just gets stressful. Looking back on it, I didn't like studying for Step 1, but it really is a superb way to consolidate and refresh all that you learned during the first 2 years of med school. Good luck.