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For Meaningful Match, Total All Centimorgans, or Consider Only Biggest Segment?

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  • For Meaningful Match, Total All Centimorgans, or Consider Only Biggest Segment?

    Hello. I am brand new at genetic genealogy. Yesterday, was looking at FF atDNA matches -- two sisters -- with whom I also share an X-match. Used chromosome browser and was wondering: In considering whether a match (non-X or X DNA) is meaningful or random, are we adding up the total number of centimorgans we share with someone (all segments/blocks), or looking only at the "longest" or "biggest" segment? Is the match meaningful if all centimorgans add up to more than 10, for example? Yesterday, I had one segment at about 4, one at about 9, and many smaller. Insight appreciated. Thanks!

  • #2
    FTDNA includes very small segments for your matches, where the other companies do not. The general advice is that it's best to only regard matches who share at least 7 cM and above (for chromosomes 1-22), so if you add up all those segments which are 7 cM and above, you will get a more reasonable amount. Many times this will leave only the largest segment, but not always. For the example you give, you would cast out the the 4 cM segment and the many smaller segments, leaving only the 9 cM segment (based upon what you wrote).

    Your Family Finder match list shows the total amount of cM shared (column "Shared cM") with each match, and also the "Longest Block" column (longest segment). See "Family Finder - Matches" in the FTDNA Learning Center for more information.

    For any matches marked as "X-match," meaning the person matches you on the X chromosome: due to the nature of the X chromosome, it's best to at least double the usual minimum of 7 to 10 cM to consider it a valid X-match. You would then have 15 to 20 cM as a minimum for matches on the X chromosome. FTDNA will label a match as an X-match even if they only share a small amount on the X chromosome.

    Long ago (7 years) I made a post about resources for matching on the X chromosome. I think the links are still valid in that post.
    Last edited by KATM; 30 August 2020, 06:15 PM.


    • #3
      Total shared autosomal cM, but ONLY including segments over about 7 cM, is the best metric for estimating the degree of relatedness. There are a number of reliable charts that give you a quick way to translate that "total shared autosomal cM" value to a list of the possible relationships. The one I use is now all over the internet, easily found by doing a Google search on "DNA Detectives Autosomal DNA Statistics Chart". This is the one chart that I always keep next to my computer! I prefer first to draw out (or at least imagine) the possible family trees implied by the shared autosomal cM, and only then check to see if there might be a significan X match that would allow me to rule out some of the possible trees.

      The general approach is to use the genetic data in combination with whatever other information you have, to construct all the possible family trees that are consistent with the data, and then seek other evidence, whether genetic or not, that will allow you to rule out one or more of the trees, until there is only one tree left, presumably the right one. Then you continue gathering evidence to be sure you haven't missed something!


      • #4
        It is not either or. Look for large total with at least some large segments.