Nikki and Rosie, I couldn't agree more. It's quite annoying actually, every time an American person asks another American what nationality they are... it makes no sense. You are American. Full stop. Your grandparents, great-grandparents, etc can be whatever but as for you? you are American. You can say there are still traditions in your family running from those nationalities your grandparents (not you!) are/were, but you are not Swedish/German/South African/WHATEVER.
Funny, it makes one think America is unsure of its own identity when, on one side, there's American flags in every corner and crap like "if you are not supportive of the government you are not patriotic" happens, and on the other hand everyone has to go on showing off about how many different places they can find their "roots" in. Stupid.