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near matches

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    My DNA test results matched only 35 out of 37 markers with a suspected family member. My great grandfather and the suspected family members great grandfather were brothers. We should have a common ancestor which would be our great great grandfather. Can some one tell me why we did not match 37 out of 37 we were only off by 1 value on marker 456 and marker 576

  • #2
    Mutation happen!


    • #3
      Near matches

      I would not be concerned about that. It happens alot in my surname project.


      • #4
        Originally posted by rmasseyt217
        Can some one tell me why we did not match 37 out of 37 we were only off by 1 value on marker 456 and marker 576
        This is how mutation works ... you will find it explained in various places, but I attempt to explain it in prose that I find easier to understand.

        Things change at random, and the rate of change is different for different markers: typically, depending on the marker, somewhere between 1 in 500 and 1 in 250 change every generation. So, from your great-great-grandfather to you are 4 generations, and somewhere between 1 in 125 to 1 in 60 markers are expected to have changed. The same amount, but independently, is expected to happen for your suspected relative. So, you see that you expect somewhere between 1 in 60 and 1 in 30 differences between the two of you. Now, when randomly you expect 1 in 50 differences on average, as iin this case, and you choose 37 markers at random, you obviously will not see exactly 37/50 differences (you cannot have thrre-quarters of one marker difference!); when one calculates, there is actually about even chances that you see no changes at all, about 1 in 3 that you see one change, about 1 in 8 that you see two changes, and about 1 in 20 that you see more than that. I have made the simplifying assumption that markers never change back, and that you and your suspected third cousin did not get the same change on their markers by chance, but these are small effects.

        In short, what you observed is like observing three heads in a row when you toss an unbiased coin (that is also 1 in 8 odds). Not very surprising, and nothing to worry about.