I joined a new list about DNA uses in genealogy. This is a place where you can ask questions and look at answers using their archives concerning family DNA - very useful. Please see one answer concerning comparisons of DNA between members of a family. Bob Jordan X-Message: #4 Date: Mon, 23 Sep 2002 07:48:19 EDT From: [email protected] To: [email protected] Message-ID: <[email protected]> Subject: Re: [DNA] DNA Question Content-Type: text/plain; charset="US-ASCII" In a message dated 09/22/02 10:01:38 PM Pacific Daylight Time, [email protected] writes: > I know absolutely nothing about DNA, other than what I have read recently on > a web site. I have a relative (maybe), with whom I think I share a common > ancestor. It would his 2nd great grandfather, as he is 94 years of age, and > my 4th great grandfather. If we both had the Genealogy DNA test, would it > tell us if we were related, and if so, approximately how many generations? > We both share the same surname and probably are related somewhere in the > past, but could the test determine if the relationship is within the last 4 > generations? The Y chromosome DNA tests can't tell you exactly how many generations ago your MRCA (Most Recent Common Ancestor) lived. Most of the time, a son's Y chromosome results will exactly match his father, but occasionally, a mutation will occur. Scientists have compared several thousand examples of father-son Y chromosome tests, so we have some idea of the average mutation rate, which is about two times in a thousand when you are looking at a specific marker. But mutations are random events, and you cannot predict when it will occur. All we know is that the more generations have elapsed, the more likely you are to encounter a mutation. So if you compared 100 pairs of men whose MRCA lived 4 generations ago with 100 pairs of men whose MRCA lived 8 generations ago, you would find more mutations in the second group. But you will find some mutations in the first group and some perfect matches in the second group. The MRCA calculator I described in a second message today gives you a RANGE of possibilities as a function of the mutation rate and the number of markers tested. The range is called the 95% confidence interval, and it is quite wide. But the calculator assumes that the two people in question are randomly selected out of a pool of men who have matching results on their Y test (they have the same "haplotype"). They may not even have the same surname -- all we know is that their haplotypes are identical. Now in your case, you are not dealing with two randomly selected people. You already have some reasons to believe that the two people have a specific common ancestor. If the test results match (or are very similar), this supports your hypothesis, but it does not prove it. If the test results are very different, then you know your hypothesis is wrong. Ann Turner [email protected] GENEALOGY-DNA List Administrator http://lists.rootsweb.com/index/other/Miscellaneous/GENEALOGY-DNA.html DNA preservation kits: http://www.dnafiler.com Bob Jordan [email protected]