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predictions and total DNA

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  • predictions and total DNA

    I have two questions and will group them together.

    On GEDmatch my daughter in law has two distant matches. The one that is listed closest has a 9.5 cM segment and at 3cM has a total of 66cM of matching small segments.
    The other has a largest segment of 7.1cM but two more are counted for a total of 17.7cM. Then a total of 79.cM of smaller segments at 3cM.

    So which match is the closer match? At some point several smaller segments will be a closer match than a single larger segment. Or is what shown likely accurate and just accept what is shown as good enough?

    I have read some matches will be closer or farther than predicted. Would a large amount of smaller segments be an indicator of being a possible closer match? I see a couple matches that have up to triple the smaller segments than other matches.

    Thanks Alan

  • #2
    Maybe if you explain why you are looking at distant matches or these two in particular it might be helpful. As far as which one is closer you won't be able to determine based on DNA alone, you would have to compare trees. At that small amount of DNA they could be anywhere from 3rd to 12th cousins.

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    • #3
      It's not recommended to consider segments below 5.0 cM when trying to determine relatedness. You can go to the "Archaic DNA Matches" page at GEDmatch - https://www.gedmatch.com/archaic1.php - and find out why.

      If you run your kit number for matches with archaic DNA at 3.0 cM, you'll probably find that you're matching ancient remains from thousands of years ago. For instance, when I run my kit for matches at 3.0 cM, I am matching a couple of remains that are over 7,000 years old. In the case of a match with 3,200 year old remains, I'm told that I share 9 segments of 3.0 cM! Even at 4.0 cM, I share one segment with this 3,200 year old person from what's now Hungary. It's only when I raise the threshold to 5.0 cM that I don't have any matches with the various ancient DNA samples in the database.

      So, I ask you given this - how reliable does it sound to regard 3.0 cM shared segments as representing a common ancestor within the last few hundred years? Even a 5.0 cM shared segment may not be IBD (identical by descent). That's why the FTDNA and 23andMe use at least 7.0 cM segments to declare a match.

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      • #4
        She has no close matches, best is maybe 3C1R. So I look at everything. I am not comparing small segments just looking at the total amounts. So she has 5 fifth cousin matches on FTDNA. Three of them have small total amounts of DNA and two have more than double of the other three. I am just wondering if that could mean anything.

        Thanks for the replies

        Alan

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        • #5
          Originally posted by 1alan1 View Post

          I have read some matches will be closer or farther than predicted. Would a large amount of smaller segments be an indicator of being a possible closer match? I see a couple matches that have up to triple the smaller segments than other matches.

          Thanks Alan
          Not necessarily.

          This is just a theory, but it might mean that one match is from a geographical area where more of your relative's ancestors were from, and perhaps the region is a bit endogamous.

          Say for example you had two Swedish gtgt-grandparent and all your other gtgt-grandparents were from one of the peninsulas in Co Kerry in Ireland.

          Another person who shares the Swedish gtgt-grandparents would be your 3rd cousin. Say the rest of their ancestry is Italian and English. It's very likely your matching segments are via the common Swedish ancestors.

          You also find a match who shares two of your Co Kerry gtgt-grandparents and is a 3rd cousin. Say they have four other Co Kerry gtgt-grandparents from the same peninsula and the rest of their ancestry is German. Your largest mutual segment is likely to be from your known mutual gtgt-grandparents. But the other short segments might be "noise" from Co Kerry.

          I've been looking at a kit of someone from a population that had a series of genetic bottlenecks (due to migration) and they share so many little pink segments with many many of their matches.

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          • #6
            ltd-jean-pull I seen this in one of her mother daughter matches. If I look at 3cm the daughter is much closer than the mother total DNA. I looked at their matches and found another cousin a 7cm showed the same thing. I suspected that the father may also been a distant cousin.

            The two I am comparing above in my first post are Korean adoptees. I suspect they are more random rather than from the same village. Of course thats my assumption.

            Anyway, just wondering if higher total DNA could mean anything if all else was equal.

            Thanks.

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