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Testing 5th cousin - best course of action?

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  • Testing 5th cousin - best course of action?

    Hi all, I'm new to this forum and to using DNA for genealogical purposes. My first project is to use it to confirm or deny my biological decendency. I know from my own primary research in official records that my great-grandfather Josephus (4 generations removed) adopted our common last name (C.) at the marriage of his mother (previously single) when he was an adult and almost getting married himself. His step-father, Cesarius C. also officially "recognised" him (as oposed to adopted although maybe in the 19th century this was the same thing?) I have strong suspicions that Cesarius was actually also the biological father of Josephus, but now I want to prove it with DNA. Unfortunately Cesarius didn't have any surviving children from his first and third marriages, so I had to go to his father, Joannes Baptiste C. and start descendancy from there. I have located a male decendant of another line of Joannes who is also 6 generations removed from Joannes, just as I am (or would be if there is a biological connection on top of the legal one.) Both our fathers are still alive so I could maybe ask them to do a DNA test as well. My simple question: for me to (dis)prove this biological line to Joannes Baptiste and hence my living 5th cousin, what is my best DNA test option and what is the most economical one? I can provide more detail in a more private conversation if needed. Thanks in advance for your thoughts. Kris

  • #2
    You need to look at cousin matching statistics at ISOGG

    The odds of matching a single 5th cousin is 10% to 30%.


    • #3
      If referring to yDNA testing, then yDNA 37 will detect if they are likely to share a common all paternal line.


      • #4
        Test the older generation first

        As your fathers are still alive, test them first. They would be 4th cousins, and so would have a much greater chance of matching in an autosomal test ... >50% probability at FTDNA, according to the ISOGG table.

        Are both you and your 5th cousin descended in a direct male line from the common ancestor? In that case, a YDNA test may be better for answering your question. See prairielad's post above.

        Also ... you said "I have located a male descendant of another line of Joannes". Do you mean that your cousin is descended from another child of Joannes (i.e. a sibling of Cesarius)? That would make Joannes your common ancestor; and as Joannes is your great-great-great-grandfather and your cousin has the same relationship to him as you do/would, then you and your cousins would be 4th cousins not 5th. And your fathers would be 3rd cousins. Apologies if I've misread what you said
        Last edited by Fern; 11 April 2018, 01:59 AM.


        • #5
          I would recommend both Family Finder and Y-DNA37 (if there is a chance for a common male ancestor) for both "fathers". Arguments "for" are presented in previous posts.

          Mr. W.


          • #6

            Thanks for the reactions. To Fern: the other child of Joannes, Jean C., is a half brother of Cesarius C.