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  • Double Cousins?

    I have an interesting match with another company and I'd appreciate opinions.

    My match is a predicted 3-5 cousin, total shared cM 40, longest segment 26. (Probably additional small segments that don't show up in the other company's calculations). We have compared our records and the only family lines in common are from a small region of small villages in the west of England. Let's call my ancestor from that region Henry Jones. There is a birth record for him with mother Ann Jones, no father's name indicated. In census records he later appears living with an uncle, Giles Jones. Both Ann and Giles are children of Thomas Jones and Sarah (let's call her Smith).

    My DNA match has Smith ancestors, and tracing both lines back we appear to have a common ancestor. Calculating the distance of the match we are 5th cousins twice removed.

    It would appear from the records that my ancestor Henry was born out of wedlock. This is bolstered by the fact that his mother left the village and went into service, while Henry stayed behind and eventually emigrated.

    It would seem to me that 40 cM shared, longest segment 26, is very large for 5th cousins twice removed. My theory is that Henry's unnamed father was a Smith or another ancestor of my match, so that we are "double cousins" and thus share an exceptionally strong match, despite the distant relationships.

    Does this make sense? Any other explanations for the size of the matching segments?

    Thanks,

    Jim

  • #2
    A 26 cM segment seems large to me for such a distant relationship, but I'm sure one of the experts will chime in.

    For comparison, I have a match at 23 and me who is a 4th cousin. Each of us has a 1st cousin marriage among the grandchildren of our 3rd great-grandparents, but through 4 different children. Our longest shared segment is 14.1 cM, but we share 6 segments, 3 of them over 10 cM and have a total of 0.81% shared DNA.

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    • #3
      Originally posted by MoberlyDrake View Post
      A 26 cM segment seems large to me for such a distant relationship, but I'm sure one of the experts will chime in.

      . . .
      I would agree that this is how it seems on the surface. But I may have chanced upon a very curious case that may throw that assumption into question.

      None of my ancestors was in the USA before 1850, so this functions as a kind of convenient discriminating factor when evaluating the possible connections with my matches who are entirely of colonial ancestry. Our connection must necessarily be BEFORE my match's immigrant ancestor.

      Also fortuitous was the chance overlap of segments shared by two descendants of a fellow named Daniel Malone b. ~1642 and died in Virginia colony. This is a particularly useful set of matches in that these two fall on opposite copies of the chromosome as compared to three known maternal connections. So I have narrowed down the field considerably to a narrow spectrum of my father's people.

      So I have two people with whom I cannot possibly have a common ancestor after 1612 or so, and most likely after 1600.

      Individually their largest segments are 16 and 8 cM, but as they are contiguous, it is plain that the amount of DNA that I inherited in this region from this ancestor born maybe 1600 is 22 cM. Fair to say, freakin' huge for a potential 10th cousin.

      I don't find it at all difficult to believe out there is another descendant of our common ancestor who has inherited a similarly grotesque and oversized segment in this section.

      And to address Barry's original question, I know that one of my matches descends from Daniel Malone twice, and that this is one of the more inbred branches of my own family tree--so much so that I daren't make more than the most tenative guess as to the specific MRCA.

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      • #4
        I would not eliminate a double cousin relation or a lot of ancestors in common from long ago filling in a few gaps, but math shows that the roll of the genetic dice could give you a fifth cousin with that much shared. I have one 4th cousin with a longest shared segment of 70 cM. Well, he matches one 4th cousin that much and with other 4th cousins (1st cousins to each other) he has a longest segment more like 30 cM and with others he does not match. So it is a roll of the dice.

        Given that you have that DNA, the odds of the 5th cousin having the same 25cM are around 18%. That is 75%, the odds of a segment of 25cM surviving to another generation, multiplied by itself 6 times for 6 generations.

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