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Dna match is a X-match in the list but not in Chromosome browser?

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  • Dna match is a X-match in the list but not in Chromosome browser?

    Hello, can anybody help me with this? I have several X-matches in my list of dna matches, who don't show up as X-match when I compare them in the chromosome browser (on the ""X"" there is no color that belongs to that person).
    Does anybody know how I should interpret this? Is it a real X-match or not? Thanks, Liesbeth

  • #2
    The X segments for these matches of yours are probably too small to be "real"; in other words, they are not IBD (Inherited by Descent).

    You may be viewing the match in the Chromosome Browser using the default setting of 5 cM. You can change that setting (a drop menu above the selected names of the matches you are comparing) to show a smaller cM comparison (1+ or 3+ cM). The matching X segment should show at 1+ cM setting.

    Although FTDNA marks matches as being an "X-match," almost always it is too small in size to be inherited. The current advice is that an X segment probably needs to be at least 15 cM, if not 20 cM, to really indicate a connection; in other words, matching X segments need to be about two times the size that a similar autosomal segment would be (7 to 10 cM minimum), to be worth investigating.
    Last edited by KATM; 24 February 2018, 05:25 PM. Reason: changed "usual" to "current" in second sentence of last paragraph


    • #3
      Also, I think X matches should be disregarded if they are not also an autosomal match. Even if you somehow managed to get the same piece of an X chromosome as someone else who is not also an autosomal match, you would have no way to work out where that X came from.


      • #4
        Thank you both KATM and John McCoy for your answers. Yes I tried with other default setting (>1CM) and indeed they do appear now. Good to know that these small X-matches are probably not real and not very useful.


        • #5
          I really don't think FTDNA should label matches as X-matches unless they share a segment at least 15 cM long. It only confuses people who are new to DNA testing.


          • #6
            I agree with Moberly Drake that X matches should not be shown unless 15 cM. I might be slightly more tolerant. 9 or even 7. But all those 1 to 4 cM X matches are just wrong to show.

            I have 44 X matches. Only 3 are above 7 cM. One is my mother, so really 2. And just two more greater than 5 cM. Its a waste of a column. I realize women get a lot more X matches but not sure how many of them are above 7 or 15 etc.

            For some reason, I do have more X matches at 23andMe and also GedMatch. I guess because they are not tied to an autosomal match. Some of them are quite decent. Many over 20, even 30 cM.

            I would get rid of the X column entirely. Or post the actual cM instead of a super generic 'X-Match'. That would be useful And it would be nice to sort the column by size of cM, which does not seem to be the case to me.

            Actually, I think a Tab, like for the Paternal and Maternal would be nice. (And Tabs for Y and mtDNA??)
            Last edited by mabrams; 12 April 2018, 08:09 PM.


            • #7
              My take on the situation is that X matches are virtually useless unless there is also a very strong autosomal match. Let's suppose that you have a 30 cM X match and NO autosomal match. How do you proceed? Since the relationship must be at least on the order of 3rd cousins or even more remote, due to the absence of autosomal matching segments, the X match could come from almost any ancestor, except for those that can be ruled out based on your gender and the way the X chromosome is inherited. But there are still too many possibilities, unless you have other information. You will be no further ahead than if you had no X match at all.

              Also, the mention of GEDmatch rings a bell. The number of X matches that females will find on the One-to-Many report in GEDmatch is grossly inflated. Most of them, sometimes around 90 percent, will not be scored as matches with the X One-to-One comparison tool. I'm not sure what the One-to-Many matching algorithm is doing, but it is definitely not working well for females on the X chromosome! Beware!


              • #8
                One thing nobody ever seems to mention is the fact that part of the X chromosome recombines with the Y Chromosome.

                This is the reason why those segments are so short. The x-match itself, in this case, isn't useful, but the total cM match, including the X portion of that match, still is. The only time, that I will concede, where short matches don't matter, is in communities where not so distant relatives intermarry, such as my Prince Edward Island side of the family, or my wife's Ashkenazi Jewish side, because then those small pieces are being passed down multiple ways and makes estimating the relational distance difficult.

                The key thing is to remember that it is in fact possible for you to inherit part of your X chromosome from your mother's father's father. Your mother's father's X partially recombined with his Y, so one of your mother's X's contains a portion of her father's Y, and then her two X's recombine and you likely get some of that passed onto you.

                The same holds true for your father. Part of his X and Y recombined, so if you are male, you received part of the X he got from his mother in your Y chromosome, and of course the rest of the Y from him, but even his Y was a combination of his father's X and Y. If you are female, you still receive part of the Y that your father got from his father.

                Moral of the story is, the X match doesn't filter out the part of the X that recombines with the Y, so it is not a very accurate indicator of anything.

                Oh, and here's a video that shows how the X/Y recombination is different than the other chromosomes!