If you're dad is a general dentist, then your best option is probably to ask him which specialist he refers the most patients to, then go and spend a day or two with that specialist and ask him to write your letter for you. No specialist (with the exception of periodontists) would pass up on an such a good opportunity to earn major points with the guy that is sending him so many patients. In the end, its a win-win situation...you get a good LOR and a chance to shadow a specialist for a few days, and he gets more referrals from dear old dad(or mom).