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Effect of step - relationships on shared centimorgans

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  • Effect of step - relationships on shared centimorgans

    On most documentation I am the third cousin once removed of Greg, ie my great great great grandparents on one line of ancestry are Greg's great great grandparents on one line.

    This is supported by the fact that we share 40 centimorgans with a longest block of 12.

    However, there are some sources that indicate that we may not have had a common male ancestor, ie that my great great great grandfather was a different person from Greg's great great grandfather.

    Could the fact that Greg and I share 40 centimorgans be consistent with having only one common ancestor (the female) rather than two?

    I would be grateful for any comments on this matter.

  • #2
    Possibly, however you and Greg would be 3rd cousins once removed. 3rd cousins once removed generally have a range of 0-175cM shared between them with an average of ~48cM. However it is possible that 3rd cousins may not share centiMorgans at all with each other due to the nature of recombination.

    For example I have a 3rd cousin twice removed who does not show up on FamilyFinder because of FTDNAs algorithm, however we share a common direct male lineage and are exact Y-chromosomal matches. Yet through another direct male cousin who is also an exact Y-DNA match I share autosomal cM with him and his daughters who would be my 4th cousins once removed (with one daughter I share 44 and with the other I share 63 cMs).

    If you and Greg share a direct male line a Y-DNA test could confirm your relationship or confirm the possibility of there being a different biological father or not.


    • #3
      Bottom line here, autosomal matching of two kits by itself (i.e., without more evidence, perhaps from other relatives) is not usually sufficiently sensitive to distinguish 3rd cousins from 3rd cousins once removed. Either relationship could produce the observed level of matching. If there are any relatives of either of you from an earlier generation who can be tested, you might be able to make a somewhat stronger case.


      • #4
        If the female is your only common ancestor then that would make you half cousins. Are you, Greg, or some other men the direct male descendant of these ancestors you are investigating? If so, I'd recommend doing the YDNA test with a level of 37 or higher.