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Please help with family relationship puzzle

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  • Please help with family relationship puzzle

    I was hoping someone could throw light on a cousin-grandparent relationship puzzle. The question is, based on shared autosomal-DNA, do three cousins share the same grandparents? Let's say first cousin Rory shares 826 cM DNA with first cousin Seamus. First cousin Rory also shares 363 cM DNA with first cousin-once removed Mary. But cousin Seamus and cousin Mary's father, Patrick, were half-siblings (same mother, two brothers from same family as fathers). Meanwhile Seamus and Mary share 1024 cM of DNA. Do these figures tell us anything about whether (a) the three brothers (Rory's dad, Seamus' dad, Patrick's dad) shared the same mother and father, or as we suspect, (b) Rory's and Seamus' dad had a different father from Patrick's dad? Patrick's dad left the family at some point, and there were rumours that a local constable was the father of the younger brothers.

  • #2
    Estimating Relationships Based on Autosomal DNA

    The amount of inherited autosomal DNA can vary greatly from cousin to cousin. I have a match who is 4th cousin, once-removed to me and two other people. Same relationship to all three of us. But she shares 6 cM with one, 42 cM with another, and 68 cM with me.