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Chromosome Mapping

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  • Chromosome Mapping

    I thought I had a handle on this but need some help. I found 2 people in common at the same place and the other 2 people at that same location were also in common. Identified one set as being my Dad's DNA so the other set was my Mother's. And then.....I hit some matches not so simple

    for instance Let's call a place on Chromosome 17 spaces 1-20

    Adam occupies spaces 1-20
    Bertha occupies 1-10 and is "in common with" Adam
    Carl occupies 11-20
    Dennis occupies 11-20
    Carl and Dennis are "in common with" each other and Adam but not Bertha.
    Are these all from the same parent?

    Now I have
    All 3 occupy spaces 9-14 on this chromosome. I assume since none of them are "in common with" the above people they have to be from the other parent, right? The thing is none of them are "in common with" each other....Earl, Frank and George are lone rangers.

  • #2
    You can't tell who Earl, Frank and George are related to because they are not in common with either side. Just because someone is not in common with one parent doesn't mean they are in common with the other side.

    I'm guessing that Earl, Frank and George are related at fairly low cMs. Doesn't this get into IBS vs IBD?

    Also if Earl, Frank and George were related to each other, then things should be different. I would say in that case they should be related to the other (non-Adam/Bertha) side.

    Other opinions?


    • #3
      Keep in mind that FTDNA has a minimum requirement for two individuals to be a match. These numbers have been quoted as approximately a minimum largest segment of 7.7 cMs AND a total segments shared of at least 20 cMs. The ICW portion is apparently at locations 11 - 20 *with you*. Bertha, at 1 - 10 doesn't overlap any of that. Earl, Frank, and George only overlap 11 - 14, so a fairly small segment.

      There are several things you cannot tell from this. If one of your parents has tested, you'd be able to see if all of these folks match you on the same side of your family tree, which is only one component of the calculations. The other thing you cannot see is how each of them match each other. Just because 20 is the ending point *with you* and Carl, Dennis, and Adam does not mean that they don't extend to 25 or 30 or beyond. Likewise, recombination may have cut off Bertha's match at 10 with all of you, while she may share an entirely different segment with Adam that you cannot see because you do not also share this segment. Because FTDNA doesn't allow you to see how others match each other your options are to contact each of them and ask them to share with you how they overlap with one another and/or ask each of them to upload to GEDmatch where you can do a one-to-one on each of them.

      Having tested my mom and 2 sisters, I've been able to see many spots where these scenarios have played out with identified relatives. Ultimately, comparing trees is a good idea.