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X Chromosome Inheritance Theory

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  • X Chromosome Inheritance Theory

    I have a theory that I may have inherited non-recombined X chromosome DNA and I would like to know if it is valid. The validity of my theory bears directly on establishing the identity of my maternal grandfather.

    I am a male adoptee. I found my natural mother in 1991. She never knew her father, or anyone who did, other than her mother. She has no pictures and no specific information about him. In short, his identity is unknown to her.

    I took the AncestryDNA autosomal test in 2012 and transferred my data to Family Tree DNA in 2015. Over time, my AncestryDNA matches began to reveal Italian ancestry, which did not make sense in the context of my known ancestors. Through research and collaboration with some of these Italian matches, I identified an Italian immigrant family and a living female descendant who took the FTDNA Family Finder test at my request. The woman was born three years before my mother and did not know her father, although she did learn his identity after he died. Her father had three brothers who were living at the time my mother was born. That said, I speculate she is a half-sister of my mother through my maternal grandfather.

    The test results returned by FTDNA Family Finder show a close match with me:

    Relationship: 1st Cousin, Aunt/Uncle, Niece/Nephew
    Shared centimorgans: 879
    Shared segments: 28
    Longest block: 108
    X-Match: yes

    The FTDNA Chromosome Browser reveals that we share two X chromosome segments of 89.88 cM and 98.61, for a total of 188.49. If the total X chromosome is 196 cM, then we share better than 97% of one of her X chromosomes. I understand that women inherit an exact copy of their father’s X chromosome. Half-sisters with a common father will each have a copy of his X chromosome.

    I am interested in the opinions of FTDNA forum members who understand the science better than I do (probably everyone). Does this Family Finder result indicate X chromosome non-recombination from my maternal grandfather to me? Are there other viable explanations for this result?

  • #2
    Non-combined X-DNA from a grandparent is certainly possible. I have seen two first cousins (a male and a female) whose mothers are sisters and they match for 196cM on the X-chromosome.

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    • #3
      Possible yes, my two sisters are fully identical on X (using Gedmatch show this, Full base pair matching). That is they match on their entire paternal X and on their entire Maternal X.

      Each received my mother full maternal X.

      Attachment is my two sisters to my maternal Grandmother
      Attached Files

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      • #4
        X Chromosome Inheritance Theory

        I am adding a PNG attachment of the FTDNA Chromosome Browser report for review. The FTDNA prediction of "1st Cousin, Aunt/ Uncle, Niece/ Nephew" leads me to believe we are between 3 and 4 degrees of separation.
        Attached Files

        Comment


        • #5
          Originally posted by CareyAllenWood View Post
          I am a male adoptee. I found my natural mother in 1991. She never knew her father, or anyone who did, other than her mother. She has no pictures and no specific information about him. In short, his identity is unknown to her.
          You have made it half way there then, congratulations on finding your mother. I was in this same situation (father not known) until recently. I hope if she is able, that you tested her in Ancestry and FTDNA in order to be able to determine who your maternal matches are, vs. not maternal.

          Originally posted by CareyAllenWood View Post
          I identified an Italian immigrant family and a living female descendant who took the FTDNA Family Finder test at my request. The woman was born three years before my mother and did not know her father, although she did learn his identity after he died. Her father had three brothers who were living at the time my mother was born. That said, I speculate she is a half-sister of my mother through my maternal grandfather.
          edit.. just noticed.. yes half sister of your mother sounds right.

          Originally posted by CareyAllenWood View Post
          The test results returned by FTDNA Family Finder show a close match with me:

          Relationship: 1st Cousin, Aunt/Uncle, Niece/Nephew
          Shared centimorgans: 879
          Shared segments: 28
          Longest block: 108
          X-Match: yes
          That looks more like a once removed to a half sibling (half niece, half nephew, half aunt, half uncle) or full cousin.

          Originally posted by CareyAllenWood View Post
          The FTDNA Chromosome Browser reveals that we share two X chromosome segments of 89.88 cM and 98.61, for a total of 188.49. If the total X chromosome is 196 cM, then we share better than 97% of one of her X chromosomes. I understand that women inherit an exact copy of their father’s X chromosome. Half-sisters with a common father will each have a copy of his X chromosome.
          Well.. you can't be sure you share just one of her X chromosomes (maternal side or paternal side only), you may still have her grandmother and her grandfather's sides here, if she is a cousin that is. Just to ask, you know she is not matching your maternal side correct?


          Originally posted by CareyAllenWood View Post
          I am interested in the opinions of FTDNA forum members who understand the science better than I do (probably everyone). Does this Family Finder result indicate X chromosome non-recombination from my maternal grandfather to me? Are there other viable explanations for this result?
          Not necessarily unless you gather more information, or more tests, however, as already mentioned, it is possible. The X does not have to recombine, as other chromsomes do not have to either, but usually they do. If they do not, then of course one side (paternal or maternal) will not be shared on that side. I'm enclosing my son's comparisons to my wife's parents for an added example in this discussion. See chromosomes 11, 17, 20, 22 and X.

          Attached Files
          Last edited by mkdexter; 7th November 2016, 12:44 PM.

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          • #6
            I went back and re-read your post. For some reason I thought you found a match on your unknown father's side but now I see it was to your mother's side.

            Yes this person is probably your aunt, half sister to your mother based on the 879cM and the X.

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            • #7
              It is always possible that the X match (two segments, missing only a small piece to be a complete X) is the result of a double cross-over. It is also conceivable, I suppose, that there is a data problem in the intervening segment. That could be checked by looking at the raw data, to see if there are an unusual number of no-calls in the missing segment. If the segment has data, I would guess a double crossover is the most likely explanation. It seems very unlikely to me that the two very large matching X segments are the result of recombination and inheritance from some other combination of relatives.

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              • #8
                Adding GEDmatch X-DNA Comparison

                I have uploaded the AncestryDNA and FTDNA data for myself and the woman in question respectively to GEDmatch in an attempt to discover any no-calls in the "intervening segment" as suggested by John McCoy. So far I have not figured out how to extract that information, but I did utilize the GEDmatch X-DNA Comparison tool which generated the output captured in the attached PNG. If I am not misunderstanding, GEDmatch sees a single segment of 196.0 cM and a 100% match between us. I realize this is an FTDNA forum but is there a reason why I should not trust the GEDmatch result? Also, I would appreciate any hints on how to identify the no-calls using whatever tools are available. - Thank you for your comments.
                Attached Files

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                • #9
                  Is your biological mother able or willing to test?

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                  • #10
                    Each web site uses slightly different matching criteria, or, for all I know, very different criteria. If GEDmatch scores the entire X chromosome as a match, I would tend to accept that result and explain the apparent gap shown by FTDNA as a data quality issue, micro-deletion, or some other minor mishap that doesn't detract from the genealogical implications of a full match.

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                    • #11
                      My natural mother and even my maternal grandmother could test if they chose to. Unfortunately for my research, they prefer to pretend I don't exist and so I doubt I will ever have an opportunity to confirm the truth of my X chromosome inheritance. In any case, I'm satisfied that my theory is sound as far as it goes. Thank you again for your input. This has been educational.

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                      • #12
                        At least you know half your tree, if not the DNA tests. If you happen to have any 2nd cousins or closer in the FTDNA or Ancestry test - that might help you narrow down common ancestry on your father's side even without your maternal side DNA results.

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                        • #13
                          mkdexter, I do know my natural father's identity, although he died before I could find him. I did not mention him in my original post because I could not have inherited my X chromosome from him. He left me two half-brothers and one of them has taken the AncestryDNA autosomal test, for which I am the administrator. My goal here was to discover if I have a basis to believe that the 100% X chromosome match I share with the woman (my mother's half-sister?) can only be reasonably explained if she shares the exact same X chromosome with my mother and I received it without recombination from her. In this case, they must share the same father because they do not share the same mother. That man must be my grandfather. Mystery solved.

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