It's also pretty irrelevant. As far as I'm concerned if your parents were born in country X and you were born there too then that is what you are. I don't see the point in saying you're 50% German, 40% Irish and 10% Moldavian and whatnot just because some of your ancestors were. Not to mention the fact that obviously your genetic heritage gets further and further diluted with every generation and it's not like this happens in clear-cut 50/50 splits or something.
Stuff like that is interesting when browsing through your family tree on a rainy sunday afternoon, but sociologically and genetically it means fuck-all for most people. The exception being people who live in a close knit community with many others with the same heritage that retain their homeland traditions and also marry and breed mostly amongst themselves like the Italian-American community in New York or something. Those people still identify with their "homeland" so much that they would consider themselves Italian even if they are born in the US. That doesn't apply to the vast majority of people in America though.