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Designating level of cousinship?

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  • Designating level of cousinship?

    I don't see the logic in the numbers associated with these levels of cousinship. Is there something else taken into account that I don't know about? Thanks, Roy.

    Shared cM Longest Block
    4th Cousin 44.53 27.66
    4th Cousin 34.31 22.47
    3rd Cousin 50.16 21.89
    3rd Cousin 47.81 21.80
    4th Cousin 44.48 21.42
    4th Cousin 39.82 20.65
    4th Cousin 38.66 20.10
    4th Cousin 34.35 20.05
    4th Cousin 42.53 19.91
    4th Cousin 42.54 19.21
    3rd Cousin 46.43 18.56
    3rd Cousin 45.52 18.13
    4th Cousin 37.37 18.03
    4th Cousin 40.91 17.68
    3rd Cousin 50.96 17.38
    4th Cousin 65.83 16.59

  • #2
    Originally posted by royfarnol View Post
    I don't see the logic in the numbers associated with these levels of cousinship. Is there something else taken into account that I don't know about? Thanks, Roy.

    Shared cM Longest Block
    4th Cousin 44.53 27.66
    4th Cousin 34.31 22.47
    3rd Cousin 50.16 21.89
    3rd Cousin 47.81 21.80
    4th Cousin 44.48 21.42
    4th Cousin 39.82 20.65
    4th Cousin 38.66 20.10
    4th Cousin 34.35 20.05
    4th Cousin 42.53 19.91
    4th Cousin 42.54 19.21
    3rd Cousin 46.43 18.56
    3rd Cousin 45.52 18.13
    4th Cousin 37.37 18.03
    4th Cousin 40.91 17.68
    3rd Cousin 50.96 17.38
    4th Cousin 65.83 16.59
    Maybe they use a very arbitrary cutoff point for the combination of the two indices. All of your third cousins have a shared cM greater than 45.0 Only one fourth cousin does but with a lower longest block.

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    • #3
      Originally posted by josh w. View Post
      Maybe they use a very arbitrary cutoff point for the combination of the two indices. All of your third cousins have a shared cM greater than 45.0 Only one fourth cousin does but with a lower longest block.
      Because of adjustment for Ashkenazi endogamy, I have a fifth cousin with a shared cM of 150 and third cousins with shared cM of 80.
      Last edited by josh w.; 17 May 2014, 10:06 AM.

      Comment


      • #4
        Originally posted by josh w. View Post
        Because of adjustment for Ashkenazi endogamy, I have a fifth cousin with a shared cM of 150 and third cousins with shared cM of 80.
        and a third cousin with 50 shared cM.

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        • #5
          I believe the total number of segments, their sizes(cM),and SNP Total all come into play.
          ie)3rd cousin
          CHROMOSOME CENTIMORGANS MATCHING SNPS
          2 1.38 500
          3 1.85 500
          3 2.81 700
          5 1.93 500
          6 1.5 4000
          6 1.57 500
          6 1.52 600
          7 2.04 500
          8 2.52 500
          8 2.2 500
          9 19.52 2795
          12 2.46 500
          17 2.61 500
          20 1.38 500
          45.29 13095SNPs

          4th cousin2 1.38 500
          3 2.61 600
          6 1.16 1200
          6 1.48 600
          7 2.48 500
          11 1.2 500
          12 2.46 500
          14 1.48 500
          15 2.56 700
          17 2.61 500
          18 21.92 2700
          19 2.3 600
          43.64 9400SNPs

          Comment


          • #6
            @Josh W & Prairielad

            Thanks. I think you are both right.

            In my experience I get the impression that all my cousin forecasts are over-optimistic but since they seem arbitrary I suppose there are a lot of cases where the forecasts are under-optimistic?

            Comment


            • #7
              Originally posted by prairielad View Post
              I believe the total number of segments, their sizes(cM),and SNP Total all come into play.
              ie)3rd cousin
              CHROMOSOME CENTIMORGANS MATCHING SNPS
              2 1.38 500
              3 1.85 500
              3 2.81 700
              5 1.93 500
              6 1.5 4000
              6 1.57 500
              6 1.52 600
              7 2.04 500
              8 2.52 500
              8 2.2 500
              9 19.52 2795
              12 2.46 500
              17 2.61 500
              20 1.38 500
              45.29 13095SNPs

              4th cousin2 1.38 500
              3 2.61 600
              6 1.16 1200
              6 1.48 600
              7 2.48 500
              11 1.2 500
              12 2.46 500
              14 1.48 500
              15 2.56 700
              17 2.61 500
              18 21.92 2700
              19 2.3 600
              43.64 9400SNPs
              Thanks. Now it is less mysterious. Is there a source of this information.

              Comment


              • #8
                Originally posted by josh w. View Post
                Thanks. Now it is less mysterious. Is there a source of this information.
                there are a few simple explanations in FAQ's
                https://www.familytreedna.com/learn/...tions-cousins/

                https://www.familytreedna.com/learn/...ionship-range/

                https://www.familytreedna.com/learn/...sidered-match/

                I don't think there is a in depth explanation of how FTDNA's algorithm works....

                Data in my previous post was just taken from a family members chromosome browser data for a suggested 3rd and 4th cousin.
                Last edited by prairielad; 17 May 2014, 08:39 PM.

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