American, in my genes?

Over the last few years I have begun to wonder how many of our human differences are actually heritable. Specifically: if Americans, both North and South, are and are growing biologically different from the descendants those who didn’t leave their home countries.
It’s not as silly as it might first sound. We’re only beginning to scratch the full complexity of our biochemical nature, and how our nurture affects it — but it is becoming clear to science that what happens to our bodies can affect what a given gene does, from turning it off to changing its products; and, that we can pass on those effects to the bodies we create.
But how far into our individual natures does this heritability go?
Two people, even two siblings, endure the same heartbreaking war or famine, or are ruled by the same invader or dictatorship. One emigrates, the other doesn’t. Why? You can’t say there’s no biochemical component, so you can’t say there’s no genetic one. And if by emigrating, a person continues to live long enough to breed, or eats a vastly improved diet (even to the point of getting fat, with those additional biochemical changes) — and if their children breed with children of people from other regions, who also left their homes?
It will probably never be an answerable question, for a host of reasons. An immigrant’s descendants may stay in the same place for ten generations after. Even if you could isolate a population of ancient settlers, at some point (unless they are West African), they must have a wanderer in their past. Not all who wander are immigrants; look at the Buddha, or Genghis Khan. A biological factor may reappear randomly but consistently, just as homosexuals continue to be produced by heterosexuals.
It’s probably less of a discussion than a story idea (and maybe that’s my best use for it). But it leads to interesting daydreaming. Certainly it would put the growing right-wing suspicion of government in a new light — maybe Americans are so suspicious of group-minded decisions, at a biochemical level, that ever-larger numbers of us can’t abide the government that we ourselves created. Of course it wasn’t all that long ago that the left-wing was equally suspicious; you say homeschooler, I say commune. We are a fissiparous people, and have always been.
Conversely, it’s a good tonic to my own knee-jerk preconceptions. Not long ago, while driving, I heard an NPR report on Israeli settlements, in which a Palestinian farmer lamented that his family had grown olives in that place for 900 years. My first reaction, I swear, was to say aloud, “Maybe try moving? Just, you know, for a century or two?”
My parents had only been in this country for four years when I was born, but clearly, I am an American. It would be interesting to know just how deep that goes, and what it might mean. Will my children stay in this land of wanderers, or will they leave when the next frontier opens? If our industrial-food-monoculture gets wiped out by blight, will my children stick it out here, or will they be the despised Okies of some other verdant continent? Those people left in the Rust Belt, whose parents’  jobs have gone for good: what of them?