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  • Comparing with a known Second Cousin

    I've tested with Family Finder, and at 23andMe. I have a known male second Cousin who tested at 8 Chromosomes with me at 23andMe, and our strongest match is Chromosome 5 with 83.3cM and 16300 SNP. He does show as a second cousin which is correct. as we share McMillan Great Grandparent's.

    Then, I tested closely with a lady trying to find her birth father. At Family Finder she shows as 2nd to 3rd Cousin, and at 23andMe she shows as 2nd Cousin. She shows as related to my known 2nd Cousin as a 3rd to 4th Cousin. At 23andMe she tested at 9 Chromosomes. From the Family Finder results, I can see our strongest match is at Chromosome 6 with 40.52 cM and 9153 SNP.

    At 23andMe they show her as related to myself, and to my known male Second Cousin. We thought she may share our McMillan and such ancestry. However, the only Chromosomes we all 3 share are at Chromosome 10 and Chromosome 11, and at 23andMe there is no overlap for those 2 Chromosomes.

    Comparing my Family Finder results for her to his 23andMe results, I do see a small overlap for 1 chromosome. At Chromosome 2 he shows 24 cM and she shows 1.69 cM where there is an overlap. I can't see other possible overlaps, since the smaller values don't seem to show up for my male Cousin at 23andMe. Also, I'm wondering how much effect the rounding at 23andMe changes the true values?

    So, it would appear to me that my male Cousin and the lady adoptee, share DNA with me from a different source? Would appreciate insight into this, as I'm somewhat confused still. I'd like to help her with the birth father issue, and want to be sure I know where we're at with this relationship between the 3 of us (or not between the 3 of us).

    Thanks so much!! Tom McMillan

  • #2
    The sorting of chromosomes into gametes is a random process, in addition to the recombination immediately previously being random process.

    Any three people (A,B,C) descended from the same set of great grandparents will, when compared pairwise, share different sets of common autosomal DNA.

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    • #3
      Comparing with known second Cousin

      I thought if the 2 parties were indeed both second Cousins with me to the same Great Grandparent's there would be overlapping of some Chromosomes? I'm wondering if that's correct or not??

      I can see a very small overlap at Chromosome 2, but unsure if that's sufficient to determine a family relationship? It seems the smaller segments (below 5 CM), aren't always accurate from what I"ve read, and the adoptee is only 1.69 CM, but has 600 SNP at Chromosome 2. My known Cousin has 24 cM and 4331 SNP at Chromosome 2.

      Then, I might be missing other small overlaps, since my known second Cousin has only tested at 23andMe where I can't see smaller segments.

      I'm somewhat technically challenged, so I'm looking for help figuring a possible relationship out, and don't want to discard a possible relationship between the 3 of us, if there might be one.

      Thanks!! Tom McMillan
      Last edited by tomwdcraftr; 30 January 2014, 12:43 PM.

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      • #4
        Whats the total cm shared?

        2nd Cousins should have around 600cm


        Originally posted by tomwdcraftr View Post
        I thought if the 2 parties were indeed both second Cousins with me to the same Great Grandparent's there would be overlapping of some Chromosomes? I'm wondering if that's correct or not??

        I can see a very small overlap at Chromosome 2, but unsure if that's sufficient to determine a family relationship? It seems the smaller segments (below 5 CM), aren't always accurate from what I"ve read, and the adoptee is only 1.69 CM, but has 600 SNP at Chromosome 2. My known Cousin has 24 cM and 4331 SNP at Chromosome 2.

        Then, I might be missing other small overlaps, since my known second Cousin has only tested at 23andMe where I can't see smaller segments.

        I'm somewhat technically challenged, so I'm looking for help figuring a possible relationship out, and don't want to discard a possible relationship between the 3 of us, if there might be one.

        Thanks!! Tom McMillan

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        • #5
          Comparing with known second Cousin

          At 23andMe my known 2nd Cousin, who shows as predicted 2nd Cousin, shows as Half IBD at 269 cM and 10 segments shared.

          At 23andMe the adoptee shows as 2nd Cousin and Half IBD at 225cM and 10 segments shared. At Family Finder she shows as 2nd to 3rd Cousin and 223.31 cM.

          My concern is currently not what level of Cousin she is to me. What I'm interested in knowing is whether the 3 of us are related by common ancestry. To be related by common ancestry I thought there should be a shared Chromosome that overlaps?? If my thinking about the shared Chromosome that overlaps is wrong, I want to adjust my thoughts.

          Right now, it would seem that the adoptee relates to me in a different way than she relates to my known second Cousin, but??

          Thanks!! Tom McMillan

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          • #6
            Do the two people of interest - the known 2nd cousin and the adoptee match each other? If so, how much.

            I was just noting today that three cousins match a more distant cousin and from the viewpoint of the more distant cousin the matches are not at all with the same segments.

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            • #7
              Comparing with known second Cousin

              They match each other 36 cM at 4 segments. They have matches at Chromosome 3; 9; 10; 15.

              At 23andMe I match my known Cousin at: 2; 3; 3; 5; 10; 11; 15; 17; 17; 20.

              At 23andMe I match the adoptee at: 1; 4; 6; 8; 10; 11; 13; 16; 19; 19.

              There is no overlapping at Chromosome 10 or 11.

              Thanks!! Tom McMillan

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              • #8
                Matching segment??

                Maybe I'm complicating my question too much.

                This might be more of a "newbie" question than advanced. It seems from what I read I'm looking for a "matching segment". Wouldn't that mean I am looking for a segment that would somewhat "overlap" to have a match??

                Thanks!! Tom

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                • #9
                  second cousin average

                  Actually, I believe that second cousins would share, on average, slightly more than 200 cM.

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                  • #10
                    that would be a 3rd Cousin


                    Originally posted by stennor View Post
                    Actually, I believe that second cousins would share, on average, slightly more than 200 cM.

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                    • #11
                      My known 2nd cousin matches me at Family Finder 168.71 total cM and 35.40 longest block. That is under 200.

                      I know her, knew her mother and her grandmother, who was a sister to my grandmother.

                      I have a higher match - 256.18 total cM 39.77 longest block. We have good GEDCOM's and have no idea how we match but it has to be "distant".
                      Last edited by GGA; 13 February 2014, 06:42 AM.

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                      • #12
                        Originally posted by GGA View Post
                        My known 2nd cousin matches me at Family Finder 168.71 total cM and 35.40 longest block. That is under 200.

                        I know her, knew her mother and her grandmother, who was a sister to my grandmother.

                        I have a higher match - 256.18 total cM 39.77 longest block. We have good GEDCOM's and have no idea how we match but it has to be "distant".
                        From your description of the relationship, which I bolded above, that does sound like a full 2nd cousin. However, I think sharing only 168.71 cM indicates it may not be a full 2nd cousin relationship.

                        I write that based on the sharing of a known 2nd cousin with me and my sister at 23andMe. The 2nd cousin shares an usually large amount of DNA with me, 472 cM. (We both must have received a large "dose" of DNA from our common great-grandparents.) My sister shares 297 cM with the 2nd cousin, more in the normal range.

                        We also have a known 2nd cousin, once removed (same common great-grandparents). I share 110 cM with her and my sister shares 82 cM.

                        Your 168.71 cM shared with your 2nd cousin is between the 297 cM my sister shares with our 2nd cousin and the 110 cM I share with our 2nd cousin, once removed. This is not even taking into account that FTDNA (where I assume you tested) counts segments less than 5 cM when the threshold is met, but 23andMe only counts segments of 5 cM or more. So, your numbers may be inflated compared to mine.

                        Either your sharing with your 2nd cousin is on the very low end or there may be a half-sibling relationship among the lines in your common ancestry with your 2nd cousin. For instance, maybe your grandmother and your 2nd cousin's grandmother were half-siblings, not full siblings. That would reduce the shared DNA of you and your 2nd cousin.

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                        • #13
                          I can't guarantee that my grandmother and her sister were full sisters , but before I had my 2nd cousin tested I had been able to confirm matches between distant relatives from both our great grandparents (parents of the two sisters).

                          I manage my 2nd cousin's kit and have been able to match her to relatives on both our great grandparents' sides.

                          Back to the original poster's question, I have been able to confirm seven matches for descendants of my 6G grandparents through their GEDCOM's and our correspondence. They don't match each other or match me in the same chromosome.

                          Granted, these 6G grandparents are my 6G grandparents twice - and 6G grandparents are distant, but the results are not what I expected.

                          My 2nd cousin only matches one of my seven matches. She has three additional matches to the same 6G grandparents and I match those three with small numbers in GEDmatch - under FTFF's criteria.
                          Attached Files
                          Last edited by GGA; 13 February 2014, 11:03 AM.

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                          • #14
                            MMaddi: Thanks for sharing those real life numbers. My known 1st cousin, 1x removed (his mother and I were 1st cousins), share about 448 cMs at 23. Is this also considered a "large dose?"

                            My grandparents are my match's great grandparents.

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                            • #15
                              23&me numbers are lower than Here but 448cm is just right for a 2nd cousin


                              Originally posted by marietta View Post
                              MMaddi: Thanks for sharing those real life numbers. My known 1st cousin, 1x removed (his mother and I were 1st cousins), share about 448 cMs at 23. Is this also considered a "large dose?"

                              My grandparents are my match's great grandparents.

                              Comment

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