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2nd cousin on father's side an X-match?

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  • 2nd cousin on father's side an X-match?

    I really don't understand the x-match. I mean REALLY don't understand it. My half- 2nd cousin and I are both females. Her grandmother and my father were half siblings(same father-different mothers). Her FATHER is my half 1st cousin. What does it mean that we are x-matches? ANY explanation fit for a 2 year old would be helpful. Thanks.

  • #2
    You need to tell us how they match on the X: how many cM is the largest matching segment? Is there more than one segment, and how big is it (or they)? To find out, select your half- 2nd cousin in your match list, and click on the Chromosome Browser button, to see the size of the match on the X chromosome. The label "X match" can be misleading, because FTDNA will show X matches regardless of how small the segment is, so you do need to check. You can disregard the X match if the segment is very small.

    In general for X-matches, you need a segment that is about twice as big as what would be considered the minimum size on another chromosome, to be considered Identical by Descent (IBD). Example: if you usually use 7 cM as a minimum for largest segment size (actually considered low nowadays), you would need at least a 15 cM segment on the X. Many people use 10 cM, or even 15 cM, as a minimum largest segment size to use for matches, so that would be 20cM or 30 cM for X matches.

    Overall on all chromosomes, your half second cousin probably shares about an average of 112 cM with you, or within a range of 30-215 cM, according to the DNA Detectives Autosomal Statistics Chart.


    Last edited by KATM; 17th February 2019, 08:10 AM.

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    • #3
      Originally posted by hbic3 View Post
      I really don't understand the x-match. I mean REALLY don't understand it. My half- 2nd cousin and I are both females. Her grandmother and my father were half siblings(same father-different mothers). Her FATHER is my half 1st cousin. What does it mean that we are x-matches? ANY explanation fit for a 2 year old would be helpful. Thanks.
      Simple possible answer is you likely had a "distant" cousin of yours marry(/find their way) into the family tree of your half-2nd Cousin. In other words, you're related to your half-second cousin more than one way.

      I had a recent "DNA Surprise" happen, and he looks to be both my half-1st Cousin, once removed and a 2nd Cousin, once removed as well. When you discover your half-Uncle fathered a child on your mother's 1st cousin(not related to the half-Uncle except by way of his step-mother), it makes for an interesting DNA puzzle to unravel. (He's a grandson from that particular pairing, his father died 15-ish years ago)

      Another example in my own family tree is we've discovered my brother's sister-in-law married someone who turns out to be a (half-)4th Cousin of ours. Because of that, I have some nieces and nephews who have the potential to have some of their 1st Cousins(the ones born to that sister-in-law) show up as related to BOTH sides of their family tree. Although as half-5th Cousins going the other way, the odds are actually reasonably good against it happening, while possible, it isn't very probable, but I've seen matches at that kind of distance -and even further- happen in the past.

      At least in their case, a "X-DNA Match" isn't likely through that specific avenue, the only common ancestor is male(and why they're half cousins), and the 4th cousin's line is the male line(and why the relationship was discovered, the surname was known to us), while our line is MFMF for us from there.
      Last edited by bartarl260; 2nd May 2019, 12:01 PM.

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