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Y-DNA and autosomal test results

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  • Y-DNA and autosomal test results

    Can someone explain how a member can come up as a Y-DNA match but not an autosomal match? One with think if a Y-DNA match was truly a match, they would show up on both if they've also done the Family Finder. Maybe that's not always the case? Can someone explain this to me?

  • #2
    A Y-DNA match can be very many generations back, as Y-DNA is passed from father to son virtually unchanged for generations, with mutations occurring rarely.

    Family Finder tests your autosomal DNA. You get 50% percent from each parent. People get an average of 25% from each grandparent, but it varies because atDNA inheritance is random. An average of 12.5% from each great-grandparent, etc. Keep dividing and remember the randomness. After a few generations, you have ancestors from whom you received no atDNA at all.

    If you test a 3rd cousin there is a 10% chance that you won't match. Even though both of you have DNA from the common ancestor, you did not receive an identical segment.

    It goes down rapidly. If you were to test 100 of your 4th cousins, only about half of them would share enough atDNA to match you on an at-DNA test.


    • #3
      To confirm and slightly expand the excellent answer from Moberly, the specificity of YDNA for a paternal line is not matched in autosomal DNA, which shows estimated relationships from each of your parents' shares to you.

      I usually encourage my Y members to do Family Finder, while some Y donors to question the value of Family Finder, because they will rarely see their surname holders or anyone they recognize at all, as atDNA shares,

      But if you sponsor or encourage a suspected cousin in your direct Y line who has a Y match to do Family Finder, you can sometimes confirm that connection if you are within four gens of the common ancestor. My 3nd gr gf's brother comes down to a male who shows as a correctly distanced cousin to me in Family Finder. This is important in our Y Project because our families use slightly different surnames (Slayton and Slatton). But if we were a gen or two back for a common male ancestor, that distance may dilute to no atDNA share at all. That is why most of my highly probable Y cousins do not show up in Family Finder, most are over six gens back to a a common male. Further, since my 2nd gr gf was a twin, it would be an excellent test project to find a descendant of that twin brother and sponsor both a Y and Family Finder to test the cM sizes from descendants of twins. I am still working on that.

      But do not expect a confirmation for your Y matches in Family Finder unless you are in the happy place where your grandfather, father, paternal uncle, brother or paternal male cousin within a few gens to a common male ancestor tests for Family Finder. Also, any high shares you have in Family Finder might by chance be a good genealogical contact (it happened to me.)
      Last edited by clintonslayton76; 3 June 2018, 04:18 PM. Reason: expansion