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Difference in # of generation to MRCA

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  • Difference in # of generation to MRCA

    I tested with ftdna FF but my brother and a known cousin tested with ancestry. We all uploaded to gedmatch and I saw something I don't understand, perhaps someone can explain it to me.

    My brother and I share 3rd great grand parents with this cousin. We have verified this through genealogical paper trails.

    When I do one to one comparisons on gedmatch between myself and my cousin and between my brother and my cousin the number of generations to MRCA come up different.

    For me with my cousin it is 5.2, for my brother with my cousin it is 3.9. I do not understand why there is a difference, shouldn't we be the same or at least closer?

    I compared another cousin (whose relationship I have not yet established) with myself and my brother and the generations to MRCA come up about the same 3.8 and 3.9 respectively.

    If anyone can tell me why there is such a difference I would appreciate the knowledge.
    Last edited by Sis65; 30 July 2017, 01:04 PM. Reason: Grammatical error

  • #2
    Are the AncestryDNA kits from later than May 2016. AncestryDNA prior to that had a kit similar to Family Finder, but the differences are greater now.



    • #3
      The estimates by GEDmatch of generations to a common ancestor are solely based on the amount of shared DNA and how many segments are shared. It's probably the case that in the example you gave your brother shares more DNA with this cousin than you do. So, GEDmatch sees your brother as more closely related to this cousin then it seems that you are. This is why these are estimates and not to be taken literally or as written in stone.

      Remember that full siblings share 50% of DNA, due to the random recombination of segments in their parents when sperm and eggs are created. This random recombination means that one child may get more than 25% of his/her DNA from the paternal or maternal grandfather, while the sibling may get more than 25% of his/her DNA from the paternal or maternal grandmother; the average that a grandparent contributes to grandchildren is 25%. This is the case with my sister and me. By comparing DNA shared with four known 2nd cousins in my paternal grandfather's line, it's obvious that I received more of my grandfather's DNA than my sister did.

      Up to about 2nd cousins, the recombination usually isn't extensive enough to cause siblings to be estimated that far apart in generations to a common ancestor with a match. When you get to 3rd cousins, it's not unusual to find that one sibling may be estimated to be a 3rd cousin, while another is estimated to be a 5th cousin to a match. Once you get to 4th cousins, there are a good number of cases where one sibling may be estimated to be a 3rd-5th cousin to a match, but another sibling doesn't share enough DNA to be declared a match.
      Last edited by MMaddi; 30 July 2017, 02:19 PM.


      • #4
        MMADDI, thanks, that does explain why I am seeing some of my matches as 4th to 6th where his are showing up as 2nd to 4th, but it does just seem to be more so on the one side of the family- mom's. On dad's side of the family it seems like the matches for both of us are about the same, the ones we have anyway. He has several matches on ancestry that I do not have on ftdna. I did have him transfer to ftdna so we can compare all of our matches; his, mine and ours.

        Jack, my brother did his back in December 2016 (ancestry) and I did mine more recently with ftdna in May. I am now anxiously awaiting mtdna. I'm certain that will open a whole world of new questions!

        So glad I discovered the forums!

        Thanks again.