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Thread: Anyone using Gedmatch for X chromosome comparison?

  1. #1
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    Anyone using Gedmatch for X chromosome comparison?

    I have just started using Gedmatch for matching across the genome, as people who have tested at FTDNA (Family Finder), 23andMe and Ancestry have posted their results here. Quite sophisticated comparisons are drawn.

    I seem to have quite a large number of matches on the X chromosome. Is this to be expected as being male, I have only 21 of 128 ancestors at the 5 x great grandparent level that can contribute?

    Looking to triangulate so I can hopefully hone in on common ancestors that way.

  2. #2
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    Hello,

    Because half of us only have one X, it's heavily selected for, in other words, there's more that can go wrong so less genetic variation.

    Some of "what can go wrong" is not too severe, like colourblindness, but some is lethal or quite dangerous.

    Given that, for an X match you should probably be prepared to ignore male matches with a longest block of of less than about 8 cM and female matches of less than about 11 or 12 cM, and though I've chosen these numbers for convenience and there's no golden rule, I would very much discourage trying to look for a common ancestor with matches much smaller than these.

    Even so, you can find real X cousins through Gedmatch. Mine seem to come mainly from my mother's mother's mother (Ireland and Germany) and mother's mother's father's mother (Ireland, Scotland, and probably the Isle of Man) with a lesser contribution from my mother's father's mother's mother (mainly Cumbria).

    As you say, matches can survive intact from farther back than autosomes because there is less mutation, particularly in an X path involving more males. Even in women, the Xs do not always recombine.
     

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    Other ancestral Y lines:

    E1b-M81 Ukraine (Ashkenazi)
    E1b-V13 England
    I1-M253 Ireland
    I2-M423 Ukraine
    R1a-L176.1 Scotland
    R1b-L584 Syria/Turkey (Sephardi)
    R1b-L20 Ireland
    R1b-L21 (1)England; (2)Wales?>Connecticut
    R1b-L48 England
    R1b-P312 Scotland
    R1b-FGC32576 Ireland

    Other ancestral mtDNA lines:

    H1b2a Ukraine (Ashkenazi)
    H6a1a3 Ukraine
    K1a9 Belarus (Ashkenazi)
    K1c2 Ireland
    V7a Ukraine

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  4. #3
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    Quote Originally Posted by AJL View Post
    Hello,

    Because half of us only have one X, it's heavily selected for, in other words, there's more that can go wrong so less genetic variation.

    Some of "what can go wrong" is not too severe, like colourblindness, but some is lethal or quite dangerous.

    Given that, for an X match you should probably be prepared to ignore male matches with a longest block of of less than about 8 cM and female matches of less than about 11 or 12 cM, and though I've chosen these numbers for convenience and there's no golden rule, I would very much discourage trying to look for a common ancestor with matches much smaller than these.

    Even so, you can find real X cousins through Gedmatch. Mine seem to come mainly from my mother's mother's mother (Ireland and Germany) and mother's mother's father's mother (Ireland, Scotland, and probably the Isle of Man) with a lesser contribution from my mother's father's mother's mother (mainly Cumbria).

    As you say, matches can survive intact from farther back than autosomes because there is less mutation, particularly in an X path involving more males. Even in women, the Xs do not always recombine.
    I wouldn't be so quick to dismiss X chromosome matches. One of the things they tell you, if you're male, is which side you're related on. It doesn't matter how small a match may be, it still has to have come from your mother's side.

    Now, that doesn't necessarily mean you're only related on your mother's side. I actually have at least a couple of cousins who are identified as paternal matches who nevertheless share with me on the X chromosome. But that's the exception.

    But here are some things that may be a bit different about my ancestry as compared to some. My parents are from very different parts of the U.S. My father's family is almost exclusively from Pennsylvania since the colonial period. My mother's family is from southern Mississippi, Oklahoma, and probably Kentucky.

    Even with the Mississippi part -- from my maternal grandmother -- 3/4 of that is "recent" immigration. (By recent, I mean after 1800.) Both her paternal grandfather and her maternal grandfather immigrated from the Mediterranean island of Minorca; her paternal grandmother was from Alsace-Lorraine. Only her maternal grandmother has deep roots in Mississippi, including indigenous ancestry.

    But, back to the X chromosome. Most of my X chromosome came from my maternal grandfather. In fact, all of it, except between about 41,030,869 and 71,279,956. That region came not only from my maternal grandmother, but specifically from her mother.

    What this means is that if I find an X chromosome match, I can tell by its location precisely which of my great grandparents I'm related through. I could, of course, have additional connections -- but I know I have at least that.

    Using Microsoft Excel, I've made a spreadsheet to which I've added over 180 names of persons who share at least one segment with me on the X chromosome. Collectively, these segments cover my entire X chromosome, from position 1 to position 155,092,794.

    Just 14 of them are related to me through my maternal grandmother's mother. This great grandmother's father was from Minorca; her mother is the one with deep roots in Mississippi. I have no Iberian (or Southern European) on my X chromosome, so I suspect this region probably does not go back to my great grandmother's father, but to her mother.

    Her mother's ancestry was French, Irish, Swiss, and Native American. One of my three sisters does have a Native American segment on her maternal X chromosome, but not at a region in which she and I match. It would still be from our maternal grandmother's maternal grandmother, but from a part I didn't inherit.

    Anyway, I haven't yet been able to determine the precise origin of the rest of my X chromosome -- the part which came from my grandfather -- other than that it's from his mother. Still, it's been my X chromosome matching that has made it possible to determine that the woman who raised my grandfather as his son was not his biological mother. Without this X chromosome data, I would still have had reason to suspect that the man who raised my grandfather was not his biological father, but I might have been more included to think that perhaps there was an infidelity on the part of my great grandmother. Instead, it seems pretty clear that it was an adoption.

    So again, the X chromosome -- and I believe even small matches -- can provide invaluable clues. At the very least, it definitely says which side a match is from -- if you're male. Even for females, though, it rules certain lines out. For example, your father's father.

    Here's one way I've been able to make use of X chromosome matching. I have a male probable 3rd cousin who actually shares three segments on the X chromosome with me. Two of these are fairly large -- 24.5 cM and 33.5 cM. Since we're both male, we can narrow this connection to our mothers immediately. I can also narrow the connection to my mother's father, and hence to his mother.

    On my cousin Dennis's side, I discovered that his mother's father has the same surname as my (probable) great grandmother's mother. So, although Dennis's mother's X chromosome could have come from either of her parents -- or both -- clearly the segments we share have to have come from a common X-path ancestor. That is, there must be an X-path from me and another from Dennis that intersects in the same individual.

    In fact, I was able to find a good candidate in both of our lines. This doesn't guarantee that I found the right person, just that I found a possible person. She not only is in both of our lines, but she is in an X-chromosome transmission path for both of us. If I remember correctly, on Dennis's side there are no female-to-female transmissions, and only one on my side.
    The short explanation of my ancestry is British-German-Catalan, but it actually includes smaller amounts of French, Irish, Swiss, Choctaw and probably Cherokee. My avatar picture is of my father, his father, and his father's father. The baby in the picture is my eldest brother.

    GB

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