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Question about differing match results between my Mother & I; why the huge disparity?

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  • Question about differing match results between my Mother & I; why the huge disparity?

    Hello all,

    I understand that I only inherit 50% of my DNA from my mother and in terms of a particular match, in the 50% I inherited, I may not have inherited those particular family bits that my Mother carries.

    However, there is a person that I will label "S" that we both match. But at completely different levels which I don't quite understand.

    My Mother is 1/2 Ashkenazi Jew and all of her matches are Jewish (840+).

    "S" is my Mother's number 3 match as a third cousin- despite her having other matches with higher numbers which is also puzzling but.....
    He is definitely not a third cousin. I have no idea if he is even an 8th cousin. But his numbers on my mother's page are: 49.58cm with a largest block of 24.16

    "S" is on my page as a 5th cousin or more way deep in my pages. Near the last page. His numbers are: 24.53cm with a largest block of 10.81

    Can someone explain the disparity? We share other matches but they are basically the same.

  • #2
    It doesn't seem like such a stretch to me that, due to recombination, the total length of shared segments was cut in half and the longest segment was cut by about 55%. Due to random recombination, it's not guaranteed that segments shared in one generation will be preserved at all in the next generation.

    Maybe others who've looked closely in the chromosome browser at shared segments across two generations have a better idea of how unusual what you note is. But I think it's easily in the realm of possibility.

    Comment


    • #3
      As Mmdaddi was saying, the size of the inherited blocs btw you and your mother seems as expected. Because of recombination, the blocks can be cut, or disappear altogether. From your example, it is not clear whether your 10.8cm common block was part of the 24cm block of your mother (which was cut during recombination) or a different, smaller block which you inherited whole. Likely, the second possibility, that is, you lost the whole big block.

      I am not familiar with the algorithm used by FF, but the longer the largest block, the more likely the relation. When blocks are small,they can be passed down for many generations without being cut, which could lead to spurious estimates. As you've probably noticed, the estimates become very noisy after a few generations. 5th cousins are basically noise, 4th could be something, or not. 3rd typically does indicate some type of relation, but not necessarily 3rd, it could be 5th.

      Given the noisiness of such estimates, it is no surprise that because of random inheritance, the estimate changes so much between you and your mother.

      cacio

      Comment


      • #4
        Yes, you answered your own question in your first sentence.

        Comment


        • #5
          Originally posted by mollyblum View Post
          Hello all,

          I understand that I only inherit 50% of my DNA from my mother and in terms of a particular match, in the 50% I inherited, I may not have inherited those particular family bits that my Mother carries.

          However, there is a person that I will label "S" that we both match. But at completely different levels which I don't quite understand.

          My Mother is 1/2 Ashkenazi Jew and all of her matches are Jewish (840+).

          "S" is my Mother's number 3 match as a third cousin- despite her having other matches with higher numbers which is also puzzling but.....
          He is definitely not a third cousin. I have no idea if he is even an 8th cousin. But his numbers on my mother's page are: 49.58cm with a largest block of 24.16

          "S" is on my page as a 5th cousin or more way deep in my pages. Near the last page. His numbers are: 24.53cm with a largest block of 10.81

          Can someone explain the disparity? We share other matches but they are basically the same.

          49.58cm = 5th Cousin
          24.53cm = 6th Cousin

          Comment


          • #6
            Originally posted by mollyblum View Post
            ...
            "S" is my Mother's number 3 match as a third cousin- despite her having other matches with higher numbers which is also puzzling but....
            FTDNA's relationship prediction algorithm is biased toward largest segment rather than total cM. You can select from the match window tool bar to change the sort to total cM.

            Comment


            • #7
              It's not unusual at all to see segment changes like that from one generation to the next. It's the luck of the draw in recombination.

              In my family's own DNA comparison I've seen(from mother to son):
              63.4cM, 63.8cM and 77.1cM segments drop off completely
              only 7.2cM of a 93.6cM segment passed down

              Comment


              • #8
                Going off topic a bit, but the original question has been addressed.

                I'm surprised this match is being projected as a 3rd cousin if your mother's results are being adjusted for Ashkenazi ancestry. I wonder if the match is not being recognized as AJ. I'm also surprised your mother has no non-Jewish matches at all. I'm half-AJ too, apparently recent European on the non-AJ side, and I have around a dozen non-Jewish matches post-Build 37 upgrade; it's possible that a few more will be restored. . .time will tell.

                Anyway, I don't have any matches with longest block that's quite as long, but my two projected AJ 3rd cousins have longest blocks of 21.46 and 18.11 and total shared of 110.13 and 94.81.

                On the non-AJ side, my three closest matches are all designated as 4th cousins. Longest blocks are between 20 and 22 and total shared ranges from 31.93-40.52

                I think the biggest single flaw in FTDNA's method of projecting relatedness is the way total shared is calculated, since it includes 1-5 cM segments. Especially for an endogamous population, the chance that these segments are meaningful is very low. The approach often results in overly optimistic projections. 23andMe uses 5 cM, and that seems to be a more sensible approach.

                For the non-Jewish matches above, the one with longest block has the highest total shared. She also has the most total segments - 10 – but all except the longest are under 5 cM and only one other is over 3. The one in the middle is almost surely the closest with a second longest block of over 11. The projection for that one - 4th cousin is likely pretty close. The others are almost surely considerably more distant; they match each other, have a common surname. One is Swiss. The other has Swiss ancestors who came to America in the 1700s, so I'm guessing they're in the 6th-8th range.

                Originally posted by mollyblum View Post
                Hello all,

                I understand that I only inherit 50% of my DNA from my mother and in terms of a particular match, in the 50% I inherited, I may not have inherited those particular family bits that my Mother carries.

                However, there is a person that I will label "S" that we both match. But at completely different levels which I don't quite understand.

                My Mother is 1/2 Ashkenazi Jew and all of her matches are Jewish (840+).

                "S" is my Mother's number 3 match as a third cousin- despite her having other matches with higher numbers which is also puzzling but.....
                He is definitely not a third cousin. I have no idea if he is even an 8th cousin. But his numbers on my mother's page are: 49.58cm with a largest block of 24.16

                "S" is on my page as a 5th cousin or more way deep in my pages. Near the last page. His numbers are: 24.53cm with a largest block of 10.81

                Can someone explain the disparity? We share other matches but they are basically the same.

                Comment


                • #9
                  For those who are not Ashkenazi and, consequently, don't have it noted on their match page, FTDNA corrects their relationship prediction algorithm for those they have identified as Ashkenazi via the ancestry estimation scheme that drives Population Finder.

                  The reason for the correction is that Ashkenazi are a highly endogamous population with a great degree of background sharing that would distort relationship prediction unless discounted.

                  Comment


                  • #10
                    The correction or adjustment is usually insufficient, IMO. They do not apply the adjustment to matches where one person has been identified as AJ and the other has not.

                    Originally posted by tomcat View Post
                    For those who are not Ashkenazi and, consequently, don't have it noted on their match page, FTDNA corrects their relationship prediction algorithm for those they have identified as Ashkenazi via the ancestry estimation scheme that drives Population Finder.

                    The reason for the correction is that Ashkenazi are a highly endogamous population with a great degree of background sharing that would distort relationship prediction unless discounted.

                    Comment


                    • #11
                      Originally posted by tomcat View Post
                      For those who are not Ashkenazi and, consequently, don't have it noted on their match page, FTDNA corrects their relationship prediction algorithm for those they have identified as Ashkenazi via the ancestry estimation scheme that drives Population Finder.
                      So this is what drives that Middle Eastern survey that some of us are asked every so often for PF? I know when I questioned them about the survey I was told it is because Middle Eastern populations look very similar due to intermarriage. When I asked further questions they indicated that it was to determine if you are Jewish. And they left it at that.

                      I for the record never knew of any Jewish ancestry. I do have several Jewish matches in FF. (Much more before Build 37.) Though not hundreds or a thousand matches of them. I do not have anything on my homepage noted about correction for Ashkenanzi. Yet I have been asked the survey three times since July of last year.

                      Comment


                      • #12
                        I'm a little confused about the survey as well. I never noticed it, but it was recently called to my attention. I had the notice about Ashkenazi ancestry before I answered, but once I did, my estimated Middle Eastern percentage actually declined somewhat.

                        My half brother, who is fully AJ, is identified in PF as 55-45 European-Middle Eastern, and his Middle Eastern is designated Druze, Palestinian. He's not being adjusted for AJ ancestry, which is annoying but informative in terms of how the adjustment operates. AFAIK, there's never been a request that he fill out a questionnaire.

                        Originally posted by 1796 View Post
                        So this is what drives that Middle Eastern survey that some of us are asked every so often for PF? I know when I questioned them about the survey I was told it is because Middle Eastern populations look very similar due to intermarriage. When I asked further questions they indicated that it was to determine if you are Jewish. And they left it at that.

                        I for the record never knew of any Jewish ancestry. I do have several Jewish matches in FF. (Much more before Build 37.) Though not hundreds or a thousand matches of them. I do not have anything on my homepage noted about correction for Ashkenanzi. Yet I have been asked the survey three times since July of last year.

                        Comment


                        • #13
                          Originally posted by NYMark View Post
                          I'm a little confused about the survey as well. I never noticed it, but it was recently called to my attention. I had the notice about Ashkenazi ancestry before I answered, but once I did, my estimated Middle Eastern percentage actually declined somewhat.

                          My half brother, who is fully AJ, is identified in PF as 55-45 European-Middle Eastern, and his Middle Eastern is designated Druze, Palestinian. He's not being adjusted for AJ ancestry, which is annoying but informative in terms of how the adjustment operates. AFAIK, there's never been a request that he fill out a questionnaire.
                          Very confusing indeed! I know that the original poster's recent survey answer caused her ME to go up I think 1 point. My recent survey caused it to go down very miniscule amount like barely a 10th of a point. For your half brother being fully AJ you would think he would have Jewish listed as one of his groups! And one would think the adjustment would be applied. So informative, yes.

                          Comment


                          • #14
                            I don't recall ever taking the survey, although I have always been explicit in profiles. The notice was on the match pages from the start, and it does invite one to correct the record if wrong.

                            Comment


                            • #15
                              Originally posted by tomcat View Post
                              I don't recall ever taking the survey, although I have always been explicit in profiles. The notice was on the match pages from the start, and it does invite one to correct the record if wrong.
                              I just double checked my matches page. Nothing about it there, and I don't ever remember seeing anything about it.

                              Now I also just decided to check my Population Finder results. I have to laugh there is that darn survey for the 4th time! So should I just give them an answer already and see how it changes it? Hmm how to pick, Jewish, Caucasus, or Middle Eastern?
                              Last edited by Táltos; 25 March 2013, 12:54 PM.

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