I understand that cell DNA changes over time due to damage from a variety of sources, and that there are mistakes made during copying. What I'd like to know is, say, for the average person living in the U.S., exactly how common this damage is.
Is it possible that such errors are so common that the idea of a person's "genetic code" is really more just an ideal average of all the individual cells' DNA taken together, rather than something that many cells actually have exactly? In other words, what fraction of our cells have DNA differing from what we began with (whether the difference is apparently critical or not), and to what degree?