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  • Wide variance for 1st cousins

    Hi!

    I had both a maternal and paternal 1st cousin tested.

    One came back with an estimated relationship of 1st Cousin, Aunt/ Uncle, Niece/ Nephew, with 793 shared cM.

    The other cousin came back with an estimated relationship of 1st Cousin - 3rd Cousin with 371 shared cM.

    There's a wide variance between how close a match one cousin is versus the other. I realized there is a lot of randomness to what dna is inherited from each parent, and understand how a cousin can inherit much different dna.

    Would you say the above is normal? Or would there be any suspicion that the one cousin isn't a first cousin?

    Thanks!

  • #2
    Is it possible that the cousin with low sharing numbers is actually a 1st cousin, once removed, instead of a 1st cousin? Or perhaps that cousin is a 1/2 first cousin, meaning that only one of the grandparents is a common ancestor for you and him/her. In other words, perhaps only the grandmother is a common ancestor for you and the first cousin, with different grandfathers involved.

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    • #3
      Thanks for your reply! The person is believed to be a first cousin with both grand-parents in common, but, of course, anything is possible.

      The relationship of 1st Cousin - 3rd Cousin does fit with it being a 1st cousin, but I was just surprised at the variance with the two cousins.

      Based on the 1st Cousin - 3rd Cousin relationship and the amount of shared cM, would there be reason to believe that she might not exactly be a 1st cousin, or am I just questioning this needlessly? Should I just take it as face value that she is a first cousin?

      Thanks, much!

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      • #4
        Here's a very useful diagram on shared amounts of DNA for different relationships from ISOGG's Wiki - http://www.isogg.org/wiki/Autosomal_..._of_shared_DNA.

        However, these numbers may be relevant to 23andMe and not FTDNA. This is because 23andMe only reports segments of 5.0 cM or larger, even if the matches are known to be related. FTDNA's algorithm establishes matches based on sharing at least 20.0 cM, with the largest segment at least 7.7 cM. The key is that once that threshold is met, FTDNA counts shared segments down to 1.0 cM. So, the shared cM totals you report above may be inflated when using the chart I linked to.

        I suggest that you export the segments for these two cousins into Excel and then delete the segments below 5.0 cM. Then add up the total cM for these 5.0+ cM segments and compare that to the chart on the ISOGG Wiki. Of course, that will reduce the total cM for these two matches, but at least it will allow you to compare apples to apples using the chart. I think that not taking out the segments below 5.0 cM will mean you're comparing apples to oranges.

        Once you do that apples to apples comparison with the chart, I think you'll find that the cousin sharing just 371 cM is not actually a full first cousin, as I posted above, but some sort of less close relationship, possibly involving a 1/2 cousin possibility. A 1/2 cousin possibility means there may have been an NPE if you don't know that an ancestor had two different spouses.

        Edited to add: Here's another chart from the ISOGG Wiki showing ranges of expected shared cM for different relationships - http://www.isogg.org/wiki/IBD#Ranges...y_relationship.
        Last edited by MMaddi; 27 November 2015, 03:38 PM.

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        • #5
          This second chart I just linked to in my previous post is based on the Family Finder algorithm ("The following data has been supplied by Tim Janzen, and is based on 6761 cMs in FTDNA's Family Finder test."), so it should be the case that you can just use the cM numbers you have without making an adjustment for the sub-5.0 cM segments.

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          • #6
            In regards to the possibility of not sharing a Grandparent with your 1st cousin (371cM)

            Are you male or female?
            What about the 793 shared cM cousin?
            Is the 793 shared cM cousin your maternal or paternal cousin?
            Is their parent a brother or a sister to your parent?

            What about the 371 shared cM cousin, Male or Female?
            Is the 371 shared cM cousin your maternal or paternal cousin?
            Is their parent a brother or a sister to your parent?

            Depending on above you may be able to determine which Grandparent will be shared based on XDNA, and which type of additional testing may verify/eliminate others.

            Male cousins whos parents are brother and sister will not share on X, will only share autosomal, will not share yDNA or mtDNA. Not enough info to determine

            Male to Female cousins whos parents are brother and sister and male cousins father is the brother and female cousins mother is the sister. Cousin will not match on XDNA, yDNA or mtDNA. Not enough info to determine.

            Male to Female cousins whos parents are brother and sister and male cousins mother is the sister and female cousins father is the brother. Cousins should* match on XDNA from parents mother.

            Male cousins whos parents are brothers should match on yDNA which is indicating they share same paternal Grandfather. Use yDNA test to determine.

            Female cousin whos parents are brothers should* match on XDNA which is inherited from their paternal Grandmother.

            Female cousin whos parents are brother and sister should* match on XDNA through XDNA from parents mother

            Female cousin whos parents are sisters should* share on XDNA from both of sisters parents XDNA. Not able to eliminate one Grandparent over the other based on X.
            they will also share the same mtDNA inherited from their maternal Grandmother.

            *possibility of not matching may be due to recombination of 2 Generation transmission events and how much parents actually shared with each other initially. Can only determine shared Grandparent if there is an X match, not matching just means unable to determine shared grandparent.
            Last edited by prairielad; 27 November 2015, 04:26 PM.

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            • #7
              Hi!

              Thanks so much for the responses!

              I haven't had a chance to compare the shared segments to the charts on the ISOGG Wiki, yet, but I greatly appreciate knowing about this tool, and will do so.

              I am male, and both cousins are female. The 793 shared cM cousin is the daughter of my father's sister. The 371 shared cM cousin is the daughter of my mother's sister.

              I never paid any attention to this before, but if I click on the arrow underneath my maternal cousin, it shows "x-match", and in chromosome browser it shows 3 matching segments of 21.67 cM, 10.53 cM and 9.1 cM. So would I be correct in thinking that her mother and my mother shared the same mother? Which would still leave the paternity as a possible variant?

              Side question, now that I'm looking at what shows up when the arrow under the match name is clicked - separate from these two cousins that have been tested, I have a mystery male match showing as a 2nd/3rd cousin, with 220 shared cM. When I click the arrow underneath his name, it shows "Y: R-M269 | mt: K1a3a1" Does that give me any clues about his relationship? (For my 2 female cousins they both show N/A for both. Of course, they wouldn't have Y-DNA, but I would kinda think my maternal cousin would share mt dna with me?)

              Thanks, again!!!!

              Comment


              • #8
                Originally posted by Family View Post

                .........

                (For my 2 female cousins they both show N/A for both. Of course, they wouldn't have Y-DNA, but I would kinda think my maternal cousin would share mt dna with me?)

                Thanks, again!!!!
                You would both have to purchase a mtDNA test first.
                If tested, yes you should have the same mtDNA haplogroup inherited from your maternal Grandmother.
                mtDNA is only passed on from mothers to children(sons and daughters). Males do not pass on mtDNA.

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                • #9
                  Hi!

                  MMaddi - thanks, again, for the link to the 2 charts.

                  I downloaded the data for both cousins from the chromosome browser and then deleted the segments less than 5 cM.

                  When I do that, the maternal cousin (daughter of my mother's sister), shows 335 cM of autosomal dna - excluding x dna, and segments shared, over 5 cM, are 17.

                  For the paternal cousin (daughter of my father's sister), the total after deleting the segments < 5 cM is 776 cM, and segments shared, over 5 cM, are 30.

                  The first chart that you referenced shows a 1st cousin as sharing on average 881 cM, but the range is 83 - 1559 cM, so I don't know if the maternal cousin is truly a 1st cousin and is just on the lower end of the range, or if I should suspect that she's not a full 1st cousin? What do you think?

                  There are no other cousins on my mother's side, and just one other paternal cousin, that I don't think I can get to test.

                  Thanks so much!

                  Comment


                  • #10
                    Originally posted by Family View Post
                    Hi!

                    Thanks so much for the responses!

                    I haven't had a chance to compare the shared segments to the charts on the ISOGG Wiki, yet, but I greatly appreciate knowing about this tool, and will do so.

                    I am male, and both cousins are female. The 793 shared cM cousin is the daughter of my father's sister. The 371 shared cM cousin is the daughter of my mother's sister.

                    I never paid any attention to this before, but if I click on the arrow underneath my maternal cousin, it shows "x-match", and in chromosome browser it shows 3 matching segments of 21.67 cM, 10.53 cM and 9.1 cM. So would I be correct in thinking that her mother and my mother shared the same mother? Which would still leave the paternity as a possible variant?

                    ...........

                    Thanks, again!!!!
                    Yes each of your mothers received an X chromosome from each parent, each of their(your mothers) maternal X and paternal X's recombined to a single maternal X in each of you. Therefore the matching between you and cousin can come from either your maternal Grandfather or your maternal Grandmother or both. Can not determine if you only share one Grandparent.

                    Comment


                    • #11
                      Thanks for your reply! Hmmmm... so this doesn't give any leads as to whether both grandparents are shared or just one... bummer....

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