Is prejudice learned or innate?

I admittedly have prejudices that I hold as I believe most do. However, even though I hold these prejudices I couldn't tell you why I have some of them. An example of one of the prejudices I previously had was of a coworker. When I first met this coworker he had a very strong southern accent and used words like "y"all" and even said "over yonder" in our first meeting. I immediately made a judgment regarding this persons intelligence, I kept thinking " what in idiot" in my head. After working with him for a little while I found very quickly this simply was not the case, he is in fact one of the most intelligent people I have ever met. What causes prejudice in my mind and the mind of others. Are people really inherently ignorant of what they don't understand or haven't been exposed to or while growing up are these things learned from our peers?