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Less than 7 cM's......

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  • Less than 7 cM's......

    If I am trying to match Cousin Dorie who is likely a ninth generation descendant of our MRCA (myself being a 7th generation), is lowering the cM to 3 > acceptable?

    For example, on 23andme I do not match Dorie, but mystery Janet matches both myself, Dorie, and my Maternal Great Aunt Beulah.

    On FTDNA Dorie and myself do not match on the FTDNA Match List, BUT when I use the GEDmatch one to one comparison tool, Dorie and myself DO match as;
    Minimum threshold size to be included in total = 700 SNPs
    Mismatch-bunching Limit = 350 SNPs
    Noise Reduction Threshold = 0.85
    Minimum segment cM to be included in total = 3.0 cM

    Chr Start Location End Location Centimorgans (cM) SNPs
    1. 144,270,181. 149,763,826. 3.2, 724.
    4. 47,317,237. 55,674,315. 4.4, 837.
    8. 58,616,467. 61,778,015. 3.7, 731.

    Largest segment = 4.4 cM
    Total of segments > 3 cM = 11.3 cM
    Estimated number of generations to MRCA = 7.2

    With GEDmatch my sister Melody matches Dorie 2 times on Chromosome 1, and twice on 4 for a total of 14.3 cM's.
    And Great Aunt Beulah matches Dorie once on chromosome 6, 4.0 cM, 848 SNPs.
    We do not have access to Janet's raw data.

    Dorie and myself have a paper trail for our MRCA, Dorie's is a proven trail, and mine is most likely from certain letters and documents.

    Even though less than 7 cM's, can this be considered a match of our Relationship?
    Last edited by von Weltzin; 23 June 2014, 11:49 AM. Reason: Punctuation.

  • #2
    I'm copying/pasting something that was posted a few hours ago about lowering the threshold which is always contested, particularly by those who like to prove that in a known relationship/connection how little they may share. Anyway, hope this answers your question.

    One limitation is the IBS/IBD distinction, another is that each meiotic event or generation halves the amount of DNA we carry over from the previous generation; at MRCA gen-4 we only have any DNA in common with half of those to which we are genealogically related at that level.

    Since the point of doing segment matching is to find genealogical relationships, the likelihood, not considering other factors such as endogamy, matches with segments at or less than 7cM are going to be red herrings half the time; the further beyond MRCA gen -4 we get, the less likely a match is to be genealogical and the less likely it will be that you can locate a shared common ancestor.

    The 700 SNP's is a little fuzzier; we know segments with low SNP density have a greater chance of being IBS. In fact, if you look at mapped chromosome graphics you will see "greyed out" areas. These are areas of usually low SNP density known to be IBS across large groups like entire species. People have looked at the reverse, very high density segments as having more of a chance of being IBD, which seems logical, but observation has not confirmed this.

    GedMatch really needs to reconcile the thresholds used for the one to many with the one to one and I know they plan to do this, but you know how it is, so many chores, so little time.