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The longest block

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  • The longest block


    I have two sisters on my FF match list and I have seen their results. They share total 2614 cM and that´s ok. BUT they have the common longest block 267 cM !!
    It´s as much as parent/child share. I have seen,that usually siblings share about 150-200 cM. So,I think that something special on chromosome 1. Can anyone explain?


  • #2
    Remember that full sisters share two parents. That "longest block" could come in part from either or both parents. To get a better idea of exactly that is in that block (which segments from which parent), you would need to use a tool that distinguishes between "half" and "full" matching segments, and if possible, samples from both parents as well. For two siblings to share the entire length of chromosome 1 would indeed be a very uncommon occurrence, but not theoretically impossible. Perhaps they were just very lucky.


    • #3
      or both parents are related and contributions from both helped fill in the chromosome in question.


      • #4
        You can share a complete chromosome match with a sibling. In the vast majority of cases it will be a combination of full and half segment matches.

        With my full sibling I have complete chromosome matches on 9, 10, 15, 16 and 17. On the X there is a small break and we match about 98%.

        At GEDmatch in the chromosome browser I can see that the complete matches are a combination of full and half segment matches.

        Have the sisters upload their raw DNA to GEDmatch and you will get the complete view of full and half segment matches.


        • #5
          OK, Thank you very much for all of you. I went to Gedmatch and found a random sibling pair. I compared their cromosomes and indeed - a mix of half and full identical blocks.
          I guess I got this!

          All the best!