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High X-DNA match proves to be my 3rd cousin 1R

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  • High X-DNA match proves to be my 3rd cousin 1R

    My highest X-DNA match inside Gedmatch.com shares 38.7 cM of X-DNA with me consisting of a long block that's 30.5 cM with 2404 SNPs and a shorter block of 8.3 cM with 788 SNPs (one of these cM figures was rounded up to the nearest tenth) on the default search for blocks at least 7 cM long. Although both of us had tested through Family Tree DNA's Family Finder, we didn't show up as autosomal matches to each other because our longest shared autosomal block is under their 7.69 cM reporting threshold since it's only 6.8 cM with 1052 SNPs although our total shared DNA is fairly large (48.1 cM by Gedmatch's definition and would probably be higher by Family Tree DNA's). So I'm glad we both uploaded our raw data to Gedmatch and that it has a separate X-DNA match list screen.

    The 30.5 cM block is 19.75 percent of my 195.93 cM total X-DNA so I figured this percentage along with the block being over the 23 cM size recommended for realistic matching with other Ashkenazim, due to endogamy, made it significant.

    Both of us are men so it was easier to narrow down the X-DNA pathway than it would be if one of us was a woman. Before I wrote to him, I found he had posted a family history online. His mother's father and my mother's father were born in the same town in Poland and were both Ashkenazim. I figured early on that it is likely to be where our shared ancestor lived, because my X-DNA pedigree chart shows that region and a distant region outside of Poland proper as my only possible X-DNA pathways within the past 200 years. After comparing our extensive genealogical trees, he confirmed that my great-great-grandmother was a sister of his great-grandmother, both of whom were born in that town and had the same pair of parents. That makes us third cousins once removed.

    I had been looking for that branch of my family using paper-trail genealogical research, to no avail, partly because when they Americanized their last name it was into a new form I wasn't expecting. But when I mentioned that I had found this "lost" branch of our family to my closer relatives, their memories flooded back of how they had met some of them in their home a few times around the early 1950s along with their grandmother from this X-DNA lineage. Also, a third branch of this family whom I had discovered in 2012 through JewishGen.org had been in touch with this "lost" branch more than a decade ago. For whatever reasons, none of my known relatives had mentioned that they already knew these people even though that branch of our family tree which they all received from me had incomplete information about them.

  • #2
    This is why FTDNA should let us see significant X matches who do not meet the autosomal standard. The drop off between finding 90% of 3rd cousins to only 50% of 4th cousins means missing a lot of cousins who could be traced by traditional genealogy if we are given a DNA hint to look.

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    • #3
      Based on the experience I outlined, I agree with CMMcDonald that there should be some way within FTDNA, even if it's an on/off toggle option or a separate screen, to see extra matches like mine where the longest X-DNA block is over some threshold, whether it's 25 or 27 cM or whatever seems reasonable to the administrators, and at the same time there's at least say 6 or 6.5 cM of sharing of a particular autosomal block and/or the total autosomal DNA meets a criteria such as 20 cM or higher.

      A small correction since I can't edit my top message anymore: I meant to say my longest block shared with him is 15.6 percent of my X-DNA and in all I share 19.75 percent of X-DNA with him.
      Last edited by khazaria; 31st May 2015, 08:41 AM.

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      • #4
        Originally posted by CMMcDonald View Post
        This is why FTDNA should let us see significant X matches who do not meet the autosomal standard. The drop off between finding 90% of 3rd cousins to only 50% of 4th cousins means missing a lot of cousins who could be traced by traditional genealogy if we are given a DNA hint to look.
        Maybe FTDNA's autosomal standard is too high.

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        • #5
          The drop off between finding 90% of 3rd cousins to only 50% of 4th cousins means missing a lot of cousins who could be traced by traditional genealogy if we are given a DNA hint to look.
          I can't add much, but will add that I know of a 5th cousin, once removed who compared with me on Gedmatch. We have a 6.1cm segment in common - too short to be a "match," but long enough for me to consider our paper trail believable.

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          • #6
            Originally posted by robertalabama View Post
            I can't add much, but will add that I know of a 5th cousin, once removed who compared with me on Gedmatch. We have a 6.1cm segment in common - too short to be a "match," but long enough for me to consider our paper trail believable.
            Just to be sure, 6.1 cM segment on X chromosome?

            W. (Mr.)

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            • #7
              No, autosomal. Sorry about that. I realize the main topic is X matches, but just threw the other in about distant matches who don't quite match enough to show up as a match.

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              • #8
                @robertalabama - not about X chromosome matching

                Originally posted by robertalabama View Post
                No, autosomal. Sorry about that. I realize the main topic is X matches, but just threw the other in about distant matches who don't quite match enough to show up as a match.
                Thank you for the clarification!

                This is how autosomal DNA is inherited. And not much can be done about it. If somebody has very few matches, switching to gedmatch can allow seeing those below the FTDNA threshold.

                However, if one already has thousands of matches above 7 cM (or whatever the exact threshold is), seeing next thousands of matches at gedmatch is mostly not useful. Using a lower threshold is then useful to verify common DNA with people known from the paper trail only.

                W. (Mr.)

                P.S.
                Going forward, people are starting with more and more Family Finder matches from the day one...

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                • #9
                  Large longest block on X chromosome

                  I also like the idea to have a switch (a toggle).

                  Initially, upon reading the proposal above, I had thought that for simplicity only people with a large longest block on X chromosome should be shown. But that would be confusing.

                  So the switch should additionally show people with a large longest block on X chromosome, and there should be an option to sort by longest block on X chromosome.

                  W. (Mr.)

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                  • #10
                    I agree. It's tough enough just to figure out the pretty close matches. Sorting out the really distant ones is quite tough, unless there are closer ones to compare to on the same line.

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                    • #11
                      Bennett told me today that he and his team will implement some kind of viewing ability for significant X-DNA matches who don't match by the standard autosomal limits into Family Finder a few weeks from now.

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                      • #12
                        Originally posted by khazaria View Post
                        Bennett told me today that he and his team will implement some kind of viewing ability for significant X-DNA matches who don't match by the standard autosomal limits into Family Finder a few weeks from now.
                        OMG!

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                        • #13
                          Originally posted by khazaria View Post
                          Bennett told me today that he and his team will implement some kind of viewing ability for significant X-DNA matches who don't match by the standard autosomal limits into Family Finder a few weeks from now.
                          Great!

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                          • #14
                            If they would do way with the 20cm autosomal minimum too, they could have much more positive word of mouth, and more customers.

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                            • #15
                              Originally posted by Lincoln View Post
                              If they would do way with the 20cm autosomal minimum too, they could have much more positive word of mouth, and more customers.
                              Would that be an option only for those who has less than, let's say, 1000 matches?

                              It might be a fair option for somebody who only has tens of matches, but for everybody..., I do not know...

                              W. (Mr.)

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