You double the number of potential ancestors for each generation you go back. It doesn't take very many generations for the number of your potential ancestors to nearly equal or even surpass the number of inhabitants in a given area at a given time.
This is particularly true if you are talking about pre-1670 or so colonial America. You are dealing with a relatively small pool of potential ancestors. If one of yours made it there before 1670, chances are pretty good that we're distantly related.
The gene pool begins to get mixed big time after 1670 as that is about the time the motherland discovered we were a good place to dump all sorts of undesirable types, not just religious dissidents and people actually wanting to come here.
You have two parents, so share half the genetic material with each. For a first cousin, you would share one-quarter of the same genetic material or 1/(2^2). By the time you get down to a 10th cousin, your common genetic material is 1/(2^12) or 1/4096th or 0.024%. I think Dick Cheney and BO are somewhere in this category. This would not be dissimilar to just about anyone in America with a pre-1670 ancestor.
My family was pre-1670, Massachusetts, NOT Plymouth. They settled at Martha’s Vineyard. Only person most people would know that I’m related to is the late Patrick Swayze. His family went back a LONG way in U.S. history.
” One of my relatives is listed in Wikipedia (Owen Fitzpen, very interesting story, by the way) and shares common ancestry with three U.S. Presidents.”
Actually, your relative, like Mitt Romney—if related to any president except Martin van Buren—is therefore related to ALL U.S. presidents (except van Buren).
http://weareallrelated.com/ http://www.freerepublic.com/focus/chat/2698845/posts
I’m a little surprised Ancestry.com doesn’t know this.