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Thread: I need help with the results of a DNA match.

  1. #1
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    I need help with the results of a DNA match.

    While building my family tree on Myheritage, I was contacted by a person with whom I shared a 6th generation ancestor, our 5th generation ancestors (we share both 6th generation parents) were siblings. When talking we realized that we had no DNA match on Myheritage, which caught our attention, we decided to compare our DNAs through Gedmatch, in the first instance we did not have a result until we reduced the minimum length from 7cm to 3cm. There we found that we had matches in 11 segments, but of short length, the longest being 4.3 cm, sharing a total of 35 cm.
    This situation surprised us because, being so short, the explanations for these concordances can be many and not a common ancestor of the sixth generation.
    This set off my alarms and I thought about the possibility that one of our ancestors is not the son of who he claimed to be on paper. So he turned to you to ask if you could clarify my doubts.
    Is there a possibility of sharing so many segments, but of such a short duration, our 5 generation ancestors were brothers, or is it a strange situation that can have other explanations?
    (When I refer to fifth generation ancestor I mean the father of a great-great-grandfather)
    Attached photo of our results in GEDMATCHInkedResultados ADN.jpg

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     C J Wyatt III (09-30-2022)

  3. #2
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    Based on your story, he is your 4th cousin or 5th cousin, if I look at the statistics from ISOGG, on average someone shares dna with roughly 50-75% of their 4th cousins, so the chance seems quite large, about 1 in 4 that you don't share dna with a 4th cousin, nothing strange there: both of you will likely have inherited some segments from your shared ancestor, but at different locations in your genome.
    The 3cM segments are so small that they can easily be coincidental.
    Last edited by Pylsteen; 09-30-2022 at 06:43 PM.

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    Do you have any candidates for triangulation?

    If you are able to triangulate with another line to the common ancestor, the match is good.

    The problem with short segments is the same with big segments. Who is the common ancestor? It is not always who we want it to be.
    Jack Wyatt

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    I would try make triangulation with other matches for verify it. Try this process with the larger segment first.

    If the match is correct after all, it doesn’t matter if segments are little. Autosomal recombination sometimes give situations some random.
    23andMe: 98.8% Spanish & Portuguese, 0.3% Ashkenazi Jewish, 0.9% Trace Ancestry (0.4% Coptic Egypcian, 0.3% Nigerian, 0.2% Bengali & Northeast Indian)

    My Heritage: 91.5% Iberian, 3.6% Ashkenazi Jewish, 2.7% Middle East, 2.2% Irish, Sccotish and Welsh

    [1] "distance%=1.6019"

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  6. #5
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    I would be concerned about the low SNP counts on these segments. Low cM and Low SNPs is not a good sign. Although.... if you can triangulate them, that would seemingly validate them.

    Your segment on Chr 11 with 4.7 cM and 789 SNPs seems a bit more credible. The other segments are generally very low on one or both counts.

    (Low SNP counts could also be derived from testing a MH V1 against a V2. V1 and V2 have weak overlap. Are both kits the same version? v2 started in spring of 2019 IIRC).

    Its not surprising that you are having trouble getting an autosomal match between 6th generation ancestors. Autosomal testing is very reliable through 5th generation (3rd cousin) but does tend to drop quickly after that.

    You might look into the Q matching on Tier One.

    If either or both of you can test the appropriate parent, you should do so and then see if the small segments are still being reported. Many small segments come and go.
    Last edited by MAbrams; 10-01-2022 at 03:42 PM.

  7. #6
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    Quote Originally Posted by MAbrams View Post
    I would be concerned about the low SNP counts on these segments. Low cM and Low SNPs is not a good sign. Although.... if you can triangulate them, that would seemingly validate them.
    Here is something to think about. The probablity of half matching on a single SNP is by chance is 0.875. The probabilty of half matching by chance on a hundred SNP's in a row is 0.875 to the hundred power. That calculates to one out of 629,788. If you triangulate, by chance is almost non-existant.

    So much for false positive.
    Last edited by C J Wyatt III; 10-01-2022 at 04:20 PM.
    Jack Wyatt

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    I have found for myself in myhertiage matches, ( for last 3 years ) is that Low confidence has given me zero official match............medium confidence gives me about 50% plus and a blank about 80% plus of a true match.

    this works for me and the other 2 accounts I manage


    My Path = ( K-M9+, LT-P326+, T-M184+, L490+, M70+, PF5664+, L131+, L446+, CTS933+, CTS3767+, CTS8862+, Z19945+, BY143483+, Y349970+, Y79536+ )


    Grandfather via paternal grandmother = I1-CTS6397 yDna
    Great grandmother paternal side = T1a1e mtDna

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