Grandfather seems to be cutting it off a bit early. Even 10 generations ago you have a maximum of 1024 ancestors. Assuming 30 years per generation and that all of your ancestors were already in the US, you would be descended from at most about one out of every 400 people in the US at that point. In reality it's probably an even lower portion because there are probably people who show up multiple places in your family tree and the bulk of the US's immigration happened after that.