This is now above my pay grade. From a reader:

I help administer a family surname DNA project which gathers information from persons around the world who share the same surname. Related to your “Roots” posting, we have found that if we input a Y-Chromosome DNA (male lineage) test result into the Sorenson Molecular Genealogy Foundation global DNA database http://www.smgf.org/ysearch.jspx, we have discoverd that our test participant will exactly match DNA markers for people around the world with different surnames and who are located in some of the oddest places. Note, this test represents only ONE line of an ancient genealogical chart. Your posted “Roots” reader is missing the point. The issue is not tracking descendants of the American Indian who lived 50,000 years ago, the issue is tracking the ancestry of a descendant of that ancient person….who is living TODAY. The person who lives today has thousands of ancestors living at a time 5000 to 1000 BC. If you look at any genealogical chart which dates back just to the 1600s (like mine), you will often find the same ancestor on various parts of the chart. This isnt’ necessarily an issue of “inbreeding” (although I am Southern ). Based on my applied knowledge of the issue, the results of the study sound plausible. Of course, with any scientific exercise, the output quality is highly dependent upon the input quality.