Why Do Republicans get offended that Obama called his Grandmother a " Typical White Person" How is that racist?

First and foremost. I like Obama, Am half white, half black. My father is white, my mother is black. Explain to me why I can't say my father is a typical white guy? Unless I'm black, I don't see how that's very offensive. I make black jokes too, does that make me racist against black people? If I call my father a " Regular " or " typical " White guy, I'm entitled to say that because I'm half white. Tell me why I can't say that about my father or my mother? Pls explain it to me?