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Number of Matches in Family

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  • Number of Matches in Family

    I convinced a couple to take the FF test, and they also had their daughter test. The father has 1360 matches, the mother 1127. The daughter has a similar number but has 258 matches who match neither of the parents. They are asking me how this can happen and I don't have a good answer, other than the 258 matches are not IBD. A better answer?

  • #2
    These matches that daughter has that neither parent has maybe false matches containing all IBS segments, or they may have a larger 7 to 19cM IBD segment with multiple smaller IBS segments.

    The matching Algorithm is based on multiple criteria to declare someone a match. FTDNA's Algorithm proprietary, but seems to follow the following basic requirements

    1)Longest segment of at least 7.7cM with 500SNPs or greater(files I have looked at don't have any shorter then 7.7 for longest block)
    2)When above minimum segment is found, segments of at least approximately 1cM and 500 SNP or Greater included.
    3)above two have to add up to around 20cM in Total DNA
    Not sure if this minimum of 20cm Total also needs a certain SNP total along with it

    That being said, let say one of these matches has a matching segment of 7.7cM containing 500+SNPs, as well as a bunch of small segments in the 1 to 5cM range that are also at least 500 SNPs which push total cM over the minimum total needed to be declared a match.

    This 7.7cM segment maybe IBD, while smaller ones maybe IBS.
    One parent may have this 7.7cM segment, but matching algorithm rejects showing as a match due to insufficient additional segments to bring total over the minimum total cM needed (20cM in total)

    One would have to compare parents kits to match at Gedmatch provided both parties utilise that site.

    or

    Manual compare matches file and parents using David Pikes utilities to see if matching segments are found(same segments identified in daughter to match). But this requires sharing of Raw Data files with one another.
    http://www.math.mun.ca/~dapike/FF23utils/pair-comp.php

    These matches will most likely be distant to Remote Cousins (classified as Speculative)
    Last edited by prairielad; 4 February 2015, 12:39 PM.

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