I had the opposite experience. The grad student and med students both take a neuroanatomy class and biochemistry class (I sat with the grad students, but was an MS1 at the time). I received a med school grade, not a grad school letter grade. Both classes were necessary for my PhD, but I didn't need both types of grades (the P/F was sufficient). Classes offered to just grad students (Molecular Vision, Dev. Biology, etc) were letter grades. And my med school and grad school grades are shown on the same transcript (in chronological order, which is VERY tough to read, but oh well). I guess that it just depends on the school.