<p>Many people study different topics in their undergrad and grad careers. Usually, they are similar subjects - one wishing to go into natural science or engineering grad school would pick a science or engineering undergrad (not necessarily the same one)</p>
<p>At my university it is impossible to start medicine or law as an undergrad major - you must pick another field, usually a similar one. People wishing to go to medical school often study biology, biochemistry, biophysics, neuroscience, etc., and prepare for medical school by taking the appropriate classes and exams.</p>
<p>It should be considered a major thing to switch fields dramatically - going to undergrad in one area, and doing grad in a totally different one. If you manage to gain admission, you would probably have to complete a large amount of prerequisite work before beginning on your real grad study. </p>
<p>It is entirely possible, however. One of the foremost physicists and mathematicians working today, Edward Witten, majored in history as an undergrad and recieved his PhD in physics, and is now at princeton. It should be noted though that his case is somewhat exceptional and not typical.</p>
<p>I know next to nothing about medical school admissions, but most graduate programs that I'm familiar with leading to a master's degree explicitly require little more than an appropriate linked bachelors(=undergrad) degree; obviously they would have additional standards over that, however.</p>