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What does 100 centimorgans, with the longest block being 42 mean?

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  • What does 100 centimorgans, with the longest block being 42 mean?

    I struggle to grasp scientific principles but understand enough to know the tests my Dad and I did were different. I am posting specifically about my test which was the FamilyFinder. Ive had no surprises and my closest match is for someone with whom I share 100 centimorgans, with the longest block being 42. This person comes up as a 2nd to 4th cousin in Illinois. So question ...... what exactly does this match mean. Ive researched my lines back and have not come across any of the names the match has listed. No response to email either. I would just like to understand the connection more clearly. I have met 2nd cousins and 3rd cousins and see where they fit in my genealogical lines - but I cant fathom this one. Thanks in advance.

  • #2
    With FTDNA the total DNA shared is very misleading. What you want is the total DNA shared counting only segments larger than 6.9 cM. Upload your raw DNA to GEDmatch to get a valid cM total and to see more matches from other DNA testing companies.

    That 42 cM block by itself could be 3rd cousins, or 3rd cousins once removed or half 3rd cousins or 2nd cousins twice removed.

    A matching 42 cM block means an valid Identical By Descent match.

    What matches do you share in common? What ethnic group do you share?

    Your tree or their tree could be in error.


    • #3
      You have a strong enough match where there might be something to it. Forget names because a non paternal event (NPE) is probably behind the connection. Take a StarTrek approach - look for where lines intersect in the space-time continuum.

      I found a couple of second half cousin's this way (though it turned out that they did not want to be my cousins). The match with one was sharing 56 cM with the longest segment being 25 cm and the other was 78 cM sharing and 51 cM longest. Apparently my great grandfather had what I term his cowboy years when he spent a couple of years in Texas after the death of his first wife and before the marriage to his second wife. With two children out of wedlock, no wonder he hightailed it back to Georgia.


      • #4

        Hope this is helpful to guide you to some relationship ranges. Thank you, Blaine Bettinger.
        FTDNA Customer - mtDNA - U3a1b
        Last edited by Biblioteque; 31 August 2019, 08:58 AM.