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Total shared CMs between 1st cousins

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  • Total shared CMs between 1st cousins

    I have had my mother and two of her maternal first cousins DNA tested. Each of the three are from different children of the same couple. There were seven children in the family, but I haven't tested descendants of the others yet.

    Linda matches Mike- 522cm, longest segment 96
    Linda matches Joann- 494cm, longest segment 43
    Mike matches Joann- 379cm, longest segment 65

    I have looked at several charts listing the average CM and the range of CM for first cousins and these numbers seem low to me. I would like someone objective to give me their opinion since I am trying to solve a mystery, and I may be reading something into the low numbers that isn't there.

    Thank you!

  • #2
    These are definitely low, particularly since there are three of them.

    First cousins AVERAGE 840 or so.
    First cousins once removed average around 420.
    2nd cousins average 210

    So you can see you are a bit low for first cousins but very high for second cousins. First cousins once removed is closer. But first cousins can dip low, its just that three times in a row seems odd. Blaine Bettinger has a study on ranges. If you need a link, let me know and I or someone else will post a link.


    Half-first cousins would also be like 1st cousins, once removed, so maybe you should see if either of those is a possibility.

    I should add the caveat that you might want to consider if you are sure you want to look into this and if your relatives feel the same way.

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    • #3
      Originally posted by Squigg View Post
      I have had my mother and two of her maternal first cousins DNA tested. Each of the three are from different children of the same couple. There were seven children in the family, but I haven't tested descendants of the others yet.

      Linda matches Mike- 522cm, longest segment 96
      Linda matches Joann- 494cm, longest segment 43
      Mike matches Joann- 379cm, longest segment 65

      I have looked at several charts listing the average CM and the range of CM for first cousins and these numbers seem low to me. I would like someone objective to give me their opinion since I am trying to solve a mystery, and I may be reading something into the low numbers that isn't there.

      Thank you!
      A web search for "ISOGG Statistics" will give you the known range for 1st cousins. Prior to Blaine Bettinger's crowd-sourcing project, the minimum was thought to be 7.31% or some 494 cM. A few of Bettinger's participants reported less than that, with at least one claiming just 83 cM! Very unlikely, in my view, but not dismissed as impossible. On the other end of the range, I share 816 cM with a known 1/2 first cousin, while the expected average is 425 cM and Blaine's self-reported max is 903 cM. There's not much comfort to be found in such ranges.

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      • #4
        There is definitely an NPE on my mother's side, but I just don't know what level it is at. A first cousin on the paternal side had no shared DNA at all with my mother, and there are several other reasons for my suspicions there. But these cousins are on my mother's maternal side. All three match distant cousins on the grandmother's side, but I haven't found a connection for any of them to the grandfather's side. The grandfather's line is a fairly recent immigrant line, so that by itself isn't too suspicious.

        I have looked at the recently updated information on Blaine Bettinger's website. I am speculating that they are half cousins, but it is hard to comprehend that three different fathers would be involved. Do the longest segments seem to be in line with a half first cousin?
        Last edited by Squigg; 8 July 2016, 10:29 PM. Reason: grammar

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        • #5
          Originally posted by Squigg View Post
          it is hard to comprehend that three different fathers would be involved. Do the longest segments seem to be in line with a half first cousin?
          I was trying to envision a simple scenario how three half-cousins would happen but couldn't see it in my mind.

          Most people see longer segments as being a better quality match than an equivalent amount of smaller segments. But I dont think there is an easy way to correlate segment length to cousin relationship. Perhaps someone else sees it differently.

          I suppose you might want to test some/all of the remaining cousins. Also load to Gedmatch as there are additional good tools there. Matrix. One-To-One can detect half-siblings if you test two sibling cousins. It will not detect half-cousins however.
          Last edited by mabrams; 8 July 2016, 10:58 PM.

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          • #6
            Originally posted by mabrams View Post
            I was trying to envision a simple scenario how three half-cousins would happen but couldn't see it in my mind.

            Most people see longer segments as being a better quality match than an equivalent amount of smaller segments. But I dont think there is an easy way to correlate segment length to cousin relationship. Perhaps someone else sees it differently.
            Joann is off the oldest child of seven. Mike is from fifth child. Linda is off the seventh child. There is 23 year age difference between the oldest and youngest child. If I want to confirm the relationships, should I try to test another first cousin from one of the other children or should I test a second cousin from the grandfather's side?

            The three cousins definitely match several people on the maternal side. I thought that maybe the grandmother took in children from her side of the family, but following the family in census records doesn't really reveal this scenario.

            I do have all the kits at Gedmatch, but am not great at using the tools yet.

            Thanks for the input!
            Last edited by Squigg; 8 July 2016, 11:11 PM. Reason: addition

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            • #7
              Since your dilemma involves three unusual cousins, I think you want to test cousin#4 and cousin#5 and even cousin#6. You might need to look at a lot of combinations in order to draw strong conclusions. Or you might get lucky and one more cousin might just make everything so obvious...

              Definitely test the 2nd cousin from the grandfather's side. 2nd cousins will match 98% of the time, so if you get two or three zeros, that will tell you a lot. You might make this priority number one.

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              • #8
                Additional question

                I forgot to mention the CM shared between myself and my daughter to the two cousins (the other one is my mother).

                I would be a first cousin, once removed-
                to Mike- 256cm, longest 66cm
                to Joann- 346cm, longest 41cm

                My daughter would be first cousin, twice removed-
                to Mike- 157cm, longest 47cm
                to Joann- 222cm, longest 42cm

                If this information helps with the first cousin or half first cousin question, please let me know. Thank you!

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                • #9
                  Well as you probably know, the ISOGG average values for first cousin once removed is around 420 and for a first cousin twice removed is around 210.

                  So your numbers are still low, particularly when Mike is involved. But no so much for Joann.

                  And your values for first cousin twice removed really are fairly decent.

                  So its possible that your values have just been affected by the randomness of DNA. All it takes is one relative who inherited on the low side, to affect lots of values. Although we get 50% of each parent, after that, there are no rules. Its not like we get 25% of each grandparent.

                  So maybe Mike inherited 20% or less from one of his grandparents.

                  I would still look into other possibilities. And test the paternal 2nd cousin.

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