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Shared cM, longest block, and in common with

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  • Shared cM, longest block, and in common with

    I know this is probably elementary for many on this forum, and I feel like a dolt. I do not have any close matches. My closest is slated to be 2nd-4th with shared cM of 51.24 and the longest block being 36.28. This is an x match for me.

    When I use the "in common with" feature, it gives me another 2nd-4th with shared cM of 53.62 and longest block of 17.60. This is NOT an x match.

    I made the (probably wrong) assumption that we all three shared a common ancestor within the 2nd-4th range based on this. I spent hours over several days plotting out both of their family trees back to the 32 ggg grandparents. Unless I have made a mistake (which is possible) there is not a match among them.

    I am an adoptee so I do not have anything to go back with. I'm just trying to find a common thread or line that I can trace back to the present.

    Can anyone give me explanations and where I can go from here?

  • #2
    Any three people can form a trio of related humans, but they may not share all the same ancestors.

    As you've discovered, person A and person B may be related, person B and person C may be related, and person A and person C may be related. But in all three cases we could easily be looking at three different ancestors. Additionally, people can be multiply related - sharing many ancestors.

    This is where the "segment chasing" comes in. The idea is to find many people who are related to you on the same portion of one of your chromosomes. Once you have a group of people who do that, you can look at their family trees and look for common ancestor(s) or patterns (such as geography.)

    For someone who knows nothing of their biological ancestry, starting say with 4th cousins (i.e., those with whom you share a GGG grandparent) is the hard way. Hopefully in time and with a wide net (check out you will be able to snag a first or second cousin, if not a sibling.
    Last edited by S9 H9; 11 January 2014, 03:11 PM.


    • #3
      Thanks, they both do have overlapping segments on the same chromosome, so I don't know if that matters or not. I just uploaded to Gedmatch, so hopefully more info will come.