As of end of fall 2017, about a quarter of students in typical pre-med courses earned A- or higher grades: http://talk.collegeconfidential.com/university-california-berkeley/2071932-grade-distributions-in-prerequisite-courses-for-gpa-based-goals.html
Here are some grade distribution links for other colleges: http://talk.collegeconfidential.com/pre-med-topics/2074436-some-colleges-have-grade-distribution-information-available-by-course.html