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Can DNA Confirm Hypothesis of a 5th Ceneration Relationship?

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  • Can DNA Confirm Hypothesis of a 5th Ceneration Relationship?

    I really don’t know much about the degree to which DNA tests can help solve a specific common relationship question. I am hoping I could get some guidance or at least find out what is feasible on a particular hypothesis about a deceased person. I’m a novice on all of this.

    For two living distant cousins/persons of the same generation, is there a DNA test that is specific enough in what it measures that it could prove (with near if not explicit certainty) whether or not a deceased person from exactly five generations ago was in fact a relative of both persons? Here’s the situation:

    I have long sought to find my Irish great-great-grandfather (i.e., five generations back). After years of traditional records searching for him, I recently identified a male (1800-1891) buried in Missouri for which I have some historical evidence it is likely him, but not convincingly so. By sheer dogged pursuit over the years (or luck), I came in contact with a woman out in California, of my generation, who is definitely a fifth generation descendant of this gentleman. If we both do a particular type of DNA test, is it possible for comparison of results to show common “markers”, or whatever, that state we definitely had a common ancestor exactly five generations ago? Here would be my direct ancestry trace to this gentleman, to be either proven or disproven by comparing the DNA of myself and the woman in CA:
    Me (male) --> My Mother (female)--> My grandfather (male) --> My great-grandfather (male)--> Gentleman in question (male). The CA woman, from whom I learned of this gentleman, has an indisputable direct 5 generation trace back to him involving both male and female blood relatives.

    Are there generational markers from some DNA test that can help prove or disprove the theory/hypothesis that we have a common ancestor specifically at this fifth generation level? If so, what comparative DNA test would we use to test the hypothesis that this gentleman was a great-great-grandfather to each of us? Also, would we need a lab to do the comparison and report their findings or is this just a matter of buying two “kits” somewhere and reading the directions?

    Thanks for any help or guidance on this!

  • #2
    The relevant test here would be the Family Finder Test.

    The important element here is your cousinship to the other tester.

    Family Finder has a 90% chance of detecting a relationship between 3rd cousins but only a 10% chance between 4th cousins.

    If you think you are 3rd cousins the family finder test will likely be worthwhile. If you are 4th or 5th cousins it can be quite a gamble.

    You will not get absolute proof from this test. If you don't match each other it might mean your are not related or it could be just down to how the DNA is inherited.

    For 3rd cousins if you do match it gives you some strong supporting evidence but there is always the possibility that you are related in multiple ways over the last 400 years so it's not conclusive. For this reason many people also hope to test a third know 3rd cousin to 'triangulate' the match and add additional weight to the evidence.

    The test will not be able to detect for sure if you are full third cousins but will be able to estimate something like 2nd to 4th or 4th to distant. This is because these are based on mathematical averages and real life dna does not always conform well to these averages.

    In summary you will not get proof but you might get strong supporting evidence or you might learn nothing at all.*

    Earl.

    * When I say nothing at all, I mean in connection with your current genealogical problem. Test test will of course match you with potentially hundreds of people that your are related to within the last 400 years or so (opinions vary on that timescale either way).
    Last edited by Earl Davis; 18 July 2014, 04:46 AM.

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    • #3
      Originally posted by Earl Davis View Post
      The relevant test here would be the Family Finder Test.
      The important element here is your cousinship to the other tester.
      Family Finder has a 90% chance of detecting a relationship between 3rd cousins but only a 10% chance between 4th cousins.
      Earl:

      Thanks so much! That is really helpful.

      If I'm understanding the generic "Cousins Chart" at Wikipedia(dot)org correctly, then I and the CA woman would be 3rd Cousins, assuming the gentleman in question (1800-1891) is in fact the great-great-grandfather of both of us.
      en.wikipedia(dot)org/wiki/Cousin#Cousin_chart

      Accordingly, the 90% probability ("chance") that you cite would make the recommended test seem worthwhile. I think I will pursue doing that test and ask the woman in California to join me. From your reply, it sounds like the fact that there would be a mix of males and females, on both sides, going back to this gentleman (1800-1891) as our same great-great-grandfather doesn't reduce or come into play on probability at the 90% level. Is that correct?

      I'm not sure how best to go about pursuing getting the "Family Finder Test" done, nor how to get the results interpreted for us. I'll now look into that. Any pointers on how to do so would be most welcomed.

      Thanks again!

      Bill

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      • #4
        Bill, I had a situation very similar to yours. I had a suspected ancestor an had been searching for proof for decades. I tested my father and another man who had a well documented paper trail to that ancestor. They were expected to be third cousins and the Family Finder test showed them to be within the range for third cousins. So it was a success for me.

        You can find a link to the FamilyTreeDNA order page at the top of these forum web pages. Just click on the products link. You will get a home page of your own after you order and your results will appear on that page.

        Susan

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        • #5
          Originally posted by Earl Davis View Post
          The relevant test here would be the Family Finder Test.

          The important element here is your cousinship to the other tester.

          Family Finder has a 90% chance of detecting a relationship between 3rd cousins but only a 10% chance between 4th cousins.
          The original poster's question has already been answered. However, I want to correct the statement I bolded above. It is correct that Family Finder will find about 90% of 3rd cousins who are in the database. However, the statement about only 10% of 4th cousins being found is wrong. The figure for 4th cousins I've always seen is 40-50%. That's a significant difference and makes Family Finder worth considering taking if you're trying to prove a 4th cousin relationship with someone.

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          • #6
            Originally posted by MMaddi View Post
            The original poster's question has already been answered. However, I want to correct the statement I bolded above. It is correct that Family Finder will find about 90% of 3rd cousins who are in the database. However, the statement about only 10% of 4th cousins being found is wrong. The figure for 4th cousins I've always seen is 40-50%. That's a significant difference and makes Family Finder worth considering taking if you're trying to prove a 4th cousin relationship with someone.
            MMaddi is correct. I mixed up the 10% figure for 5th cousins with 4th cousins.

            Earl.

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            • #7
              Don't want to confuse the issue unnecessarily but I just ran into this scenario nearly exactly; the match strength will be halved due to only one [GG Grandfather] vs both of that set of GG grandparents shared. They would be half 3rd cousins. It still can be a solid and verifying match. Fingers crossed.

              Bob H.

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              • #8
                I proved great-great-grandparents of my mother with a match between her and a great-great-grandson of the hypothetical couple by means of the Family Finder test. That happened to be the easy part. He and my mother shared 3 sizeable segments of DNA.

                All four 3rd great-grandparents are unknown, however. Can you ever prove complete unknowns as far back as 3rd great-grandparents by means of this test???

                (I think I may have proof for one set. I posted all my data in another thread, but no one has replied. Too much info to wade through, I think.)

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