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  • Same cousins but cousins don't match each other?

    Hi,
    Just wondering if anyone can help me. My Mum has done the Family Finder test and has submitted her data to Gedmatch.
    On Family Finder she has 7 cousins who all match her on Chr 15 with a segment of DNA around 12cM.
    Cousin A
    Cousin B (Cousin A's 1st cousin)
    Cousin C (Cousin B's Niece)
    Cousin D
    Cousin E
    Cousin F
    Cousin G.
    Cousins A-E are at Gedmatch and I have compared them all and they all share this segment between each other.

    On Family Finder they all appear on my Mum's chromosome browser as having a segment in the same place.

    But here's the thing, when I do "in common with" between them, Cousins A-E show everybody but Cousins F and G are not in common with each other despite showing Cousins A-E as being matches. How is this possible?

    Cousins A-E are on my Mum's paternal side as they do not show up on her maternal half sister's Gedmatch (and they both share maternal DNA in this region).

    This is driving me nuts! Thanks in advance.

  • #2
    F and G just may not share enough DNA with Cousins A to E.

    For FTDNA to declare someone a match, I believe, they need to share around at least 20cM in total.

    If F and G only share this segment on 15 of around 12cM, it is not enough by itself for FTDNA to declare them a match.

    You would have to ask them to upload to Gedmatch, or if they will share their raw data for this segment to do a independent comparison via manually or using a Utility like ones developed by David Pike.

    http://www.math.mun.ca/~dapike/FF23utils/strand.php
    http://www.math.mun.ca/~dapike/FF23utils/pair-comp.php

    Edit
    See following post which is somewhat similar, specifically post 9 which I describe above type of scenario of a segment that is present, but not declared a match due to insignificant Total DNA shared
    http://forums.familytreedna.com/showthread.php?t=36358
    Last edited by prairielad; 1 January 2015, 05:54 PM.

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    • #3
      Hi Prairielad,
      I never knew FTDNA had a total sharing threshold of 20cM!
      Looking at Mum's matches I see that her lowest match does indeed share a total of 31.16 cM the biggest segment being 7.70cM!
      So have I got this right, if F and G only share the 12cM segment and no other segments to take the match over 20cM then they will not show as being in common?
      I want to make sure I've understood correctly because this is the first line of cousins who appear to have surnames in common and they are on my Mum's unknown American paternal side (as verified by her maternal half sister's results)!

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      • #4
        Originally posted by Boudicca View Post
        Hi,
        Just wondering if anyone can help me. My Mum has done the Family Finder test and has submitted her data to Gedmatch.
        On Family Finder she has 7 cousins who all match her on Chr 15 with a segment of DNA around 12cM.
        Cousin A
        Cousin B (Cousin A's 1st cousin)
        Cousin C (Cousin B's Niece)
        Cousin D
        Cousin E
        Cousin F
        Cousin G.
        Cousins A-E are at Gedmatch and I have compared them all and they all share this segment between each other.

        On Family Finder they all appear on my Mum's chromosome browser as having a segment in the same place.

        But here's the thing, when I do "in common with" between them, Cousins A-E show everybody but Cousins F and G are not in common with each other despite showing Cousins A-E as being matches. How is this possible?

        Cousins A-E are on my Mum's paternal side as they do not show up on her maternal half sister's Gedmatch (and they both share maternal DNA in this region).

        This is driving me nuts! Thanks in advance.
        There's two sides to every chromo. You might be a match to two people on the same segment of the same chromo but person A might be a match to you on one side of the chromo and person B a match on the other side, meaning they are not a match to either other. That's why the "in common with" tool is so important.

        http://dna-explained.com/2013/12/15/...nd-the-matrix/

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        • #5
          That is correct, to be declared a match with another at FTDNA one of the criteria seems to be that you need to share at least 20cM in DNA in total. Not only will they not show as in common, they will not be shown as a match to A and E on any of their match list. Your mum just happens to share enough DNA with them all to have them on her matchlist (That is if this is an identical segment in each)

          But as Germanica states, they could very well be on your moms maternal side and just happen to share the same section along her maternal chromosome. Due to maternal and paternal chromosomes being merged in browser it just looks like they are sharing the same DNA.

          Again the only way to truly tell, is to compare on Gedmatch which has a lower matching threshold, or to manually compare Raw Data files (ie. David Pikes)

          In common with Tool is just one step in the process. It will not identify a particular segment on one single chromosome over the other.
          It is not based on matching DNA segments, it is just based on who you share in common on your match list.
          Last edited by prairielad; 1 January 2015, 07:35 PM.

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          • #6
            I see. I will try and persuade them to join Gedmatch. I have emailed them already but have yet to hear from them.
            As they are not in common with each other, could one be connected via my Mum's paternal side and the other via her maternal side?
            I think I will have to assume they are not related to Cousins A-E until they can either confirm a shared paper trail or they join Gedmatch etc.

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            • #7
              Looking at the chromosome browser, Cousin F shares from around 48000000-56500000 and Cousin G shares from around 51700000-59000000 in common with my Mum. Cousins A-E share from 47000000-59000000.
              So Cousin F and G overlap from 51700000-56500000. I have no idea what this equates to in cM but could this suggest too small a match between them?

              Comment


              • #8
                Have you tried using the Family Finder Matrix?

                Comment


                • #9
                  Originally posted by Germanica View Post
                  Have you tried using the Family Finder Matrix?
                  No what's that?

                  Comment


                  • #10
                    It's shown in the link I provided before - it shows you a matrix of multiple selected matches and whether or not they match each other. If you can't find it, it should be on your dashboard below the buttons for matches, myOrigins, etc - there's some links that say "Matrix | Advanced Matches | Download Raw Data | Learn More".

                    Comment


                    • #11
                      If you don't mind, I would like to slightly intrude on this thread to learn a little more about the match threshold.

                      Specifically, in addition to the total combined match value of 20 cM, there must be minimum value that the individual segments must have in order to be included in the total (for example, three 7-cM segments would be a match, but 50 1-cM segments would not.)

                      Could someone let me know what that value is?

                      Thanks,
                      Michael

                      Comment


                      • #12
                        Originally posted by mrayers View Post
                        If you don't mind, I would like to slightly intrude on this thread to learn a little more about the match threshold.

                        Specifically, in addition to the total combined match value of 20 cM, there must be minimum value that the individual segments must have in order to be included in the total (for example, three 7-cM segments would be a match, but 50 1-cM segments would not.)

                        Could someone let me know what that value is?

                        Thanks,
                        Michael
                        Their matching criteria and algorithm is proprietary. But by looking at the results, it seems to be declared a match you need at least the following

                        1)Longest segment of at least 7.7cM with 500SNPs or greater(files I have looked at don't have any shorter then 7.7 for longest block)
                        2)When above minimum segment is found, segments of at least approximately 1cM and 500 SNP or Greater included.
                        3)above two have to add up to around 20cM in Total DNA
                        Not sure if this minimum of 20cm Total also needs a certain SNP total along with it

                        Comment


                        • #13
                          Originally posted by prairielad View Post
                          Their matching criteria and algorithm is proprietary. But by looking at the results, it seems to be declared a match you need at least the following

                          1)Longest segment of at least 7.7cM with 500SNPs or greater(files I have looked at don't have any shorter then 7.7 for longest block)
                          2)When above minimum segment is found, segments of at least approximately 1cM and 500 SNP or Greater included.
                          3)above two have to add up to around 20cM in Total DNA
                          Not sure if this minimum of 20cm Total also needs a certain SNP total along with it
                          Thanks a lot!

                          I was wondering why a match I was looking at on GEDmatch was not showing up on FF, even though the largest matching segment is 9.1 cM and there are also 6, 6.5 , 6.7, and several smaller segments.

                          The answer appear to be that none of these segments have more than 500 SNPs (usually between 400-490)

                          So, it seems that there is no way at all I can do any comparisons with this individual here, right?

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