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  • Probabilities

    I have two exact matches at YDNA-37 with 0 distance, both of those matches coming from the same (paternal) ancestor whose surname doesn't match mine. My first question is about the probabilities for the common ancestor we share. The Tip report says 83% probability within the first 4 generations. The earliest the common ancestor could be on my side is 4 generations back, but the earliest on their side is 5 generations back. So I'm assuming I need to recalculate the probabilities in the TiP report based on the higher number (no common ancestor in the last 4 generations), which would drop the probability to 74% through 6 generations. Is that right?

    My second question is more complicated: we are getting additional matches, somewhat less exact (a couple are exact at 12 markers but off by one or two at 37), to that same surname, apparently from other branches of that family though it is hard to tell because their documented genealogy isn't well worked out. What I'm trying to figure out is what is the most likely explanation. Here I need to make fuller disclosure. Let's call my surname T. I have good documentary and circumstantial evidence that my gg grandfather is a surname G. But these Y-dna matches are to a surname R. So, to put it more crudely, is my gg grandfather carrying a G chromosome or an R chromosome? It's possible that my suspected gg grandfather G (already known to have fathered children by several women) had an illegitimate son who took the surname of the mother, which would explain the two exact matches with R. But if we are getting matches to others with that same surname R who are not direct descendants of that hypothetical illegitimate son, would that indicate that the common ancestor had to be further back in the generations? And would that in turn imply that either a) they are genetically a G but the offspring come not from my promiscuous gg grandfather but from my 3g or up grandfather; or b) am I genetically their surname R, so that my gg grandfather was the son of one of their surnamed people.

    I hope that was clear makes my head hurt a bit just to try to ask the question!