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  • X matches

    Can someone tell me what it means when it says X match on one of your matches.

  • #2
    It means you match that person on the X chromosome. Men only have one X chromosome and it comes from their mother.

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    • #3
      Beware of putting too much faith in an FTDNA match labeled as an X-match. Most of the time the segment(s) are too small to be considered more than Inheritance By State (IBS, basically by coincidence, false positive), vs. Inheritance By Descent (IBD, actually from a common ancestor). FTDNA will show a match as an X-match even if the segment is tiny. The rule of thumb is that a matching segment on the X chromosome should be at least twice the recommended minimum for the other 22 chromosomes, so instead of a minimum 7-10 cM, on the X you should only consider a minimum segment of about 15-20 cM, preferably larger.

      For further reading about the X chromosome, the ISOGG Wiki page for X-Chromosome Testing has some information, plus links to many site pages with more information. I like the pages shown in the Further Reading section by Jared Smith (X-chromosome recombination), and in the Blog Articles section by Louise Coakley (Geni 1 blog, X-DNA's helpful inheritance patterns).

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      • #4
        KATM - What a bonanza of links within links to be explored! I will bookmark all of them. So many of these links I have not seen, so will look forward to a re-education. The "Tips and Warnings About Using X -Dna" I found to be important. Thank you!

        B546846 - In case it is not easily discovered in these links, the X is Chromosome 23.

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        • #5
          Most of my x matches are very small. Mostly under 3 cM and only a few above 15 to 20. So mostly IBS

          The easiest way to scan your X matches is to download a .csv file and then sort by Chromosome. Then you can manually scan your X segments for decent size segments.


          By 'Download', I mean DOWNLOAD ALL SEGMENTS found on the upper right of the Chromosome Browser page.

          btw, females have a LOT more X matches than males. But mostly IBS


          I really really wish FTDNA would re-think the X column. Sorting would be nice. And dont make such a big deal of tiny tiny X matches. Print the actual number of cM rather than a super obvious/useless 'X"
          Last edited by mabrams; 11th August 2019, 02:27 AM.

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          • #6
            One thing nobody ever seems to mention is the fact that part of the X chromosome recombines with the Y Chromosome. This is why so many people think these matches are occurring by coincidence, when in fact they were inherited in a way that most people don't understand.

            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! https://www.biointeractive.org/class...n-y-chromosome

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            • #7
              http://www.genealogyjunkie.net/x-dna...ce-charts.html

              snetphilie, this is a good link for X Inheritance Charts for each male and female. Try all of her links which will include X Fan charts which some people prefer.
              Last edited by Biblioteque; 13th September 2019, 03:06 PM.

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              • #8
                The complication of the small segment of the X chromosome that is homologous with and therefore subject to recombination with a small piece of the Y chromosome is only relevant if the autosomal DNA test includes a series of SNP's that are part of that segment, AND includes those SNP's in its segment matching algorithm. Does anyone have the specifics to show that FTDNA or the other major vendors do or do not include those SNP's (and if so, which ones?).

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                • #9
                  Originally posted by John McCoy View Post
                  The complication of the small segment of the X chromosome that is homologous with and therefore subject to recombination with a small piece of the Y chromosome is only relevant if the autosomal DNA test includes a series of SNP's that are part of that segment, AND includes those SNP's in its segment matching algorithm. Does anyone have the specifics to show that FTDNA or the other major vendors do or do not include those SNP's (and if so, which ones?).
                  I found this thread on Pseudoautosomal regions (or PAR) from 2014, if that is of any help.

                  In answer to his own first post question,
                  Originally posted by JohnG View Post
                  Do the pseudo-autosomal regions of the Y get used in the X match?
                  JohnG later said

                  Originally posted by Ann Turner View Post
                  However, people with mother/child data at FTDNA see these numbers in the Chromosome Browser table

                  Segment boundaries: 1,370,495 to 154,570,039 (?build 36 numbers)
                  Number of SNPs 18,092
                  cM: 195.93
                  Originally posted by JohnG View Post
                  So that seems to include part of the PAR1 region at the end of the short arm tips of the X and Y, but not the PAR2 region which is at the end of the long arm tips and would be in the 155,000,000 plus range. I think it would include the PAR3 region if one has it.

                  So in the 1.37-2.4+ range one might see SNPs from a Y chromosome, I think.
                  Of course, FTDNA's chip has changed since 2014, so it may cover different regions now. It seems to me that these recombined areas of Y on the X chromosome would be such small segments as to not be relevant for genealogy.

                  Comment


                  • #10
                    FTDNA does not display PAR region matching on X.(not enough to register)
                    They give amounts in Chromosome Browser using Build 36 Data
                    Build 36 PAR regions are
                    PAR1- Position 1 to Position 2,709,520
                    PAR2- Position 154,584,238 to Position 154,913,754

                    FTDNA Chromosome browser Displays position between 1,370,495 and 154,570,099
                    Note for PAR1 only position 1,370,495 was tested, next SNP was at position 2,710,157 (PAR1 ends at 2,709,520)
                    For PAR2 no SNPs tested

                    Both ancestry and 23andme chips tested some PAR region SNPs and comparisons at Gedmatch would reflex this, some paternal sisters will show a very small segment of non matching at the beginning of the X (PAR1 Region)

                    I have not seen which SNPs FTDNA new GSA chip tests, so I do not know which PAR SNPs are now tested, but this would only be reflected when comparing two tests which were tested on the new chip, if FTDNA tests and displays positions in chromosome browser

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                    • #11
                      Thanks very much for the details, prairielad. I expected as much.

                      I was just trying to determine, in response to snetphilie's identical posts in 16 subforums back in September, if there was any practical use in his idea. Your details seem to disprove it. If the new GSA chip at FTDNA is anything like the old one, as far as what PAR SNPs are tested, then it seems we can ignore the possibility of any genealogically useful segments in the PAR regions showing up.

                      Since the segment included in the FTDNA Chromosome Browser from PAR1 has been so minimal (less than 2 cM), and doesn't even register, then it can't prove anything for genealogy. Even with using files from 23andMe or Ancestry at GEDmatch, I would think any resulting PAR1 segment is too small to be of any genealogical use. The recombined parts of the X and Y that snetphilie mentions are not as large, or in a part of the chromosome to be useful, as he might presume.

                      Originally posted by snetphilie View Post
                      One thing nobody ever seems to mention is the fact that part of the X chromosome recombines with the Y Chromosome. This is why so many people think these matches are occurring by coincidence, when in fact they were inherited in a way that most people don't understand.

                      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! https://www.biointeractive.org/class...n-y-chromosome
                      The video for which snetphilie posted the link even says
                      Most chromosomes recombine with their partners along their entire lengths, as illustrated in the first part of the animation. Modern X and Y chromosomes, however, recombine only at their tips.
                      And now we have the details from prairielad to indicate how very small these recombined segments in these pseudoautosomal regions (PAR) are, and how they are virtually not even shown in FTDNA's Chromosome Browser.

                      I do wish a moderator would remove duplicate posts such as snetphilie's, though. 16 of the identical post in as many subforums is simply overdoing it.
                      So ends my snit for today!

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                      • #12
                        To add, For those interested, PAR1 and PAR2 positions in Build 37 (Gedmatch, 23andme both use Build 37 positions)
                        PAR1 X 60,001 2,699,520
                        PAR2 X 154,931,044 155,260,560

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                        • #13
                          Thank you, prairielad, I had meant to ask if you had those figures also, for Build 37.

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                          • #14
                            Looking at FTDNA GSA Raw Data, they test 427 PAR1 SNP and 90 PAR2 SNP

                            Build37
                            PAR1 - 427 positions between 153977-2697868
                            PAR2 - 90 positions between 154938068-155234707


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                            • #15
                              But would you say that the segments in the pseudoautosomal regions are too small to interpret for genealogy, if they are even shown in a chromosome browser?

                              I know we have drifted far from the original post's question, but it's an interesting aspect of the X chromosome.

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