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  • Paging JABLAIR.........

    Howdy, as means of continuing my education re Y DNA and ensuring that I do not give potential participants erroneous information I'd like to ask you for some additional clarification regarding a response you posted to my original request for data. I had asked in a previous post if matches at 12 and 25 markers were completely negated if they failed to match at 37. Several folks seemed to state that this was not the case, i.e., only the probability was decreased. However you stated:

    Once you both have upgraded to 37 markers they are ones that determine TMRCA. The results of the 25 markers become a mute point once you both upgrade.

    The TMRCA tables for 12, 25, and 37 markers each assume that these are all the markers there are to be tested.

    The TMRCA for 12 markers assumes that there are ONLY 12 markers available for testing. The probability calculations are based on this assumption that there are only 12 markers to be tested. If there are only 12 markers and you match 12 for 12, there is a 50% probabilty that you share a common ancestor within 14 generations.

    If you both upgrade to 25 markers and match 23 for 25 there is now a 50% probability you share a common ancestor within 28 generations. The 12 marker TMRCA is no longer valid and means absolutely nothing. It is superceded by the 25 marker TMRCA table.

    If you both upgrade to 37 markers and match 32 for 37 there is now a 50% probability you share a common ancestor within 42 generations. The 12 marker TMRCA and the 25 marker TMRCA are no longer valid and mean absolutely nothing. They are superceded by the 37 marker TMRCA table.


    Admitting to it having been 1969 since I was a grad student and lightly involved in statistics, it appears to me that what you are saying makes perfectly good sense. Yes, there is a probability at 12/12 of a common ancestor; however, that also means there is a probability of no common ancestor and the only answer is more markers. Thus if further markers indicate no match then the 12/12 is "trumped" and there is likely no probability of a match. I realize that a lot of folks really want to find matches and perhaps stretch the interpretation of the data but what I don't understand is why the testing companies themselves seem to imply more significance in low resolution markers, e.g., receiving notice from them about having found another "genetic cousin" etc. Isn't this a bit disingenuous?
    Thanks for your comments.

  • #2
    Originally posted by cycleman
    I realize that a lot of folks really want to find matches and perhaps stretch the interpretation of the data but what I don't understand is why the testing companies themselves seem to imply more significance in low resolution markers, e.g., receiving notice from them about having found another "genetic cousin" etc. Isn't this a bit disingenuous?
    Thanks for your comments.
    Hi Cycleman,

    I agree. I've always felt that the testing companies were doing a disservice by not explaining what the probability tables really mean.

    At year's conference in Houston, I specifically asked if "the TMRCA for 12 markers assumes that there are ONLY 12 markers available for testing. If there are only 12 markers and you match 12 for 12, there is a 50% probabilty that you share a common ancestor within 14 generations." I was told that the probability calculations were based on this assumption that there are only 12 markers to be tested. The same holds true for probabilities based on the 25 and 37 marker tests.

    Technically they are correct when they say that "if the mutataion rate is 0.002 and you test 12 markers and if you match on all 12 markers there is a 50% probability that you share a common ancestor within 14 generations". It's simple statistics, just plug the numbers into the formula and compute the answer. And for a long time there were only 12 markers to be tested, so the underlying assumptions were basically correct. Those that matched 12/12 or 11/12 were assumed to share a common ancestor.

    Then came the 25 marker test. In most cases those that matched 12/12 or 11/12 continued to match on the 25 marker test, at least close enough to be considered related in the time frame of surnames. 12/12 usually became 25/25, 24/25, or 23/25, all still considered related, just adjusting the TMRCA. But sometimes 12/12 became 22/25 (maybe borderline) or less and the probability of sharing a common ancestor in the time frame of surnames seemed to all but disappear.

    Even 25 marker relationships can fall apart at 37 markers. And who knows, maybe someday we'll be testing 74 markers and find out that 37 marker matches fail.

    I believe that the testing companies should somehow explain this to the participants so they don't get a false sense of being related based on just 12 markers or even 25 markers or more.

    John

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