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Thread: New Ground for Me: The X Chromosome

  1. #11
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    When it comes to reproduction, the father's possibilities are either/or. That is, either he passes on a copy of his X chromosome -- without recombination; or he passes on a copy of his Y chromosome (also without recombination). This, of course, is what determines whether his child is a son or a daughter.

    For mothers, though, there are actually three possibilities. Although any child she has will receive an X chromosome from her, whether the child is a son or a daughter, that child can receive: an X chromosome which containing DNA from both maternal grandparents; an X chromosome containing DNA from only the maternal grandfather; or an X chromosome containing DNA from only the maternal grandmother.

    Which of these three things happens seems to be a matter of chance. The first scenario is the most common, but the second and third happen pretty regularly as well.

    This happened in my own family. Out of the six offspring of my parents, three received an X chromosome from our father. These are my sisters, and on that X chromosome they are identical. The other three of us received a Y chromosome from our father: i.e., my brothers and me.

    Now for reasons I'll get to below, I can tell that four of us received an X chromosome from our mother than contains DNA from both maternal grandparents, while two received an X chromosome from her than contains DNA from just one maternal grandparent. The evidence suggests that it's from our maternal grandfather.

    When I compare genomes at 23andMe, I can see that none of the three brothers matches any other brother completely on the X chromosome. Therefore, no two of us received an identical recombination of our mother's X chromosomes.

    With our sisters, this is more difficult to tell. "The New Experience" no longer distinguishes between full and half matching. Since they have the same father, my sisters have one identical X chromosome -- the one they inherited from our father. That means their genome comparison simply shows matching across the entire X chromosome.

    ("The Old Experience" did distinguish between full and half matching, so it was much easier to tell that my sisters don't match on all of their maternal X chromosome. But Gedmatch does still make this distinction, with one-to-one graphic comparisons.)

    Fortunately, I can see that each brother matches each sister differently, which means that no two sisters can have identical maternal X chromosomes.

    However, it's very important that I wrote, "no two sisters" As it turns out, my brother Bernie and my sister Kim match across the entire X chromosome. This has to be on their maternal copy of the X chromosome.

    For this to have happened, the maternal X chromosome for each of them must either have had identical crossovers, or no crossovers. The latter seems much more likely; and if there were no crossovers, then all of the DNA is from the same grandparent. (Or conversely, if all of the DNA is from the same grandparent, then there were no crossovers and the chromosome is -- effectively -- unrecombined.)

    Because Bernie and Kim have an identical maternal X chromosome, it also means that their matching with the other two brothers is identical. That is, the genome comparison between Bernie and me looks just like the comparison between Kim and me -- as far as just the X chromosome is concerned. And the same thing for Curt, of course.)

    I match both of them for all of the X chromosome except for a comparatively small region surrounding the centromere. This region is only 19.28 cM in length.

    Within the region where I don't match Bernie or Kim, the matches I share with DNA cousins have all proven to be on our maternal grandmother's side; outside the region -- on both arms -- the matches have all proven to be on our maternal grandfather's side.

    A reasonable inference is that all of Bernie and Kim's maternal copy of the X chromosome is from our maternal grandfather.
    Last edited by geebee; 02-03-2017 at 07:44 AM.
    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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  3. #12
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    Quote Originally Posted by David Mc View Post
    So this is possible? I had thought it was passed down in a straight matrilineal line (rookie that I am). Thinking about it, it makes sense I suppose, because "why not?" Our mothers carry two x chromosomes... pass one onto us... who is to say which one it will be?
    Yeah, your mom has two x chromosomes: the one she got from her mother and the one she got from her father. I have been able to eliminate my maternal grandmother as the source of my own x chromosome because an x match my mom has on that same line is not an x match for me. That leaves my maternal grandfather as the source of my x chromosome, and of course he got his x chromosome (the only one he had to pass down) from his mother. His mother was a Danley, but her mother was an English. I spotted someone among my mom's x matches with the surname English in her list of ancestral surnames, so I am thinking my ancestor Sarah Lucinda English may be the most distant known source of my x chromosome. Can't really confirm that right now, though; it may not be right.
    Last edited by rms2; 02-03-2017 at 02:41 PM.
     


    Hidden Content


    Y-DNA: R1b-FGC36982 (L21> DF13> Z39589> CTS2501> Z43690> Y8426> BY160> FGC36974>FGC36982)

    Additional Data:
    Lactase Persistent:
    rs4988235 AA (13910 TT)
    rs182549 TT (22018 AA)

    Red Hair Carrier:
    Arg160Trp+ (rs1805008 T) aka R160W

    Dad's mtDNA: K1a1

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  5. #13
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    Quote Originally Posted by geebee View Post
    . . .

    For mothers, though, there are actually three possibilities. Although any child she has will receive an X chromosome from her, whether the child is a son or a daughter, that child can receive: an X chromosome which containing DNA from both maternal grandparents; an X chromosome containing DNA from only the maternal grandfather; or an X chromosome containing DNA from only the maternal grandmother . . .
    I was not aware of that. Thanks. Obviously that complicates matters.

    What I mentioned earlier about my mother's x match with the surname English in her list of ancestral surnames turns out to be not so simple and straightforward, after all. My mother actually has a number of x matches with the surname English among their ancestral surnames, but in checking my own FF results, I find none of those people show up as x matches for me even though some of them show up as autosomal matches. So, something weird is going on, probably connected to what you were talking about.

    The x could be more trouble than it's worth.
     


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    Y-DNA: R1b-FGC36982 (L21> DF13> Z39589> CTS2501> Z43690> Y8426> BY160> FGC36974>FGC36982)

    Additional Data:
    Lactase Persistent:
    rs4988235 AA (13910 TT)
    rs182549 TT (22018 AA)

    Red Hair Carrier:
    Arg160Trp+ (rs1805008 T) aka R160W

    Dad's mtDNA: K1a1

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  7. #14
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    Quote Originally Posted by rms2 View Post
    The x could be more trouble than it's worth.
    Exactly. Once you get past about four generations, my research is showing that the large matching segments are not IBD. People really do not know what they are looking at.

    Jack Wyatt

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  9. #15
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    Quote Originally Posted by rms2 View Post
    I was not aware of that. Thanks. Obviously that complicates matters.

    What I mentioned earlier about my mother's x match with the surname English in her list of ancestral surnames turns out to be not so simple and straightforward, after all. My mother actually has a number of x matches with the surname English among their ancestral surnames, but in checking my own FF results, I find none of those people show up as x matches for me even though some of them show up as autosomal matches. So, something weird is going on, probably connected to what you were talking about.

    The x could be more trouble than it's worth.
    It can be, sometimes. It can also be a gold mine. It depends on what other family members you also have test results for. Obviously, the more the better.

    The thing to remember about the X chromosome is that some matches can be truly ancient because of how often the X manages not to recombine. Everytime it's passed from the father to daughter, of course, but also sometimes even mother to daughter or mother to son -- just as it is with other chromosomes.

    I have a niece whose paternal X chromosome is presumably identical to one that came from a great, great grandmother. Since the copy my brother got is traceable to our maternal grandfather -- and therefore to our maternal grandfather's mother -- that's the only one my niece could have received from my brother.

    So this copy of my niece's X chromosome would have come from a woman born in the 1860s. It likely is a recombination of the her two X chromosomes -- but as you can see, it wouldn't have to be. And this niece has five children to whom she could have passed this X chromosome, in entire as part of a recombination.

    Of course, it was just luck that my brother Bernie and sister Kim not only received an X chromosome entirely from one grandparent, but the same grandparent. It could have worked the other way. That would have meant they'd have 0 sharing on the X.

    So this means you can't always tell, with half siblings, which side they're on just because there is no matching on the X chromosome. Usually the X will be recombined when recombination is possible. But just as autosomes are sometimes not recombined in a generation, sometimes the X chromosome isn't recombined, even though it could have been.

    EDIT: I presume your English ancestors actually immigrated from Ireland, since that would seem like a odd name to acquire while living in England. (Sorry, but I kept help thinking of Johnny English.)
    Last edited by geebee; 02-03-2017 at 04:18 PM.
    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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  11. #16
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    A 1st cousin 1R got a match on FTDNA recently and the X is huge compared to my 1st cousin 1R X comparison.

    Chromosome Start Location End Location centiMorgans (cM) # of Matching SNPs
    1 98738916 158687887 35.81 8924
    1 167284043 169516601 2.15 800
    2 51848242 60339658 6.76 2400
    2 134027656 136533558 2.34 600
    6 721262 4662212 10.01 1488
    6 27391233 30629414 1.03 2700
    6 42366271 44419575 3.65 570
    12 8691173 21368250 17.63 4086
    12 129783638 132287718 4.83 678
    20 37420892 39568136 2.52 500
    X 22803814 79014810 55 5725


    Unfortunately both have unknown fathers so we are unable to help one another at this stage
    Last edited by Solothurn; 02-03-2017 at 04:31 PM.
    AncestryDNA link: Hidden Content

    LivingDNA link: Hidden Content

    My known ancestral lines:

    U152: A9024/BY3644 @ Vallum Aelium
    H1c3b (mother)
    U4a3 (paternal grandfather's mother)
    U198 (paternal gg grandfather)
    K1a-T195C! (paternal grandmother)

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  13. #17
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    I hope the OP will indulge what may be a bit of a digression, but I find the process by which recombination takes place to be immensely fascinating.

    Along the way, the parent's 23 pairs of chromosomes end up first being doubled, so that for autosomes 1-22 there are now four of each. These are in the form of two pairs of "identical twins". For the X chromosome, a father will have two copies of the one he received from his mother; he'll also have two copies of the Y chromosome he got from his father.

    These groups of four (or two, if there is both an X and a Y chromosome), are lined up together, and may end up exchanging parts -- I won't try to describe exactly how; but it sort of sounds like a wild party. But when recombination takes place, it can either be between two copies that are different (are from opposite parents), or two that are the same.

    That means that, technically, each chromosome is always a recombination. Even the Y is a recombination of the doubled copies of the Y chromosome. But since the two copies were identical, the recombination is also identical -- except for the odd chance that some section manages to be spliced in backwards (a reversal).

    Anyway, when this recombination has been completed, you end up with four new copies of chromosomes 1-22, and either four copies of the X chromosome (for a mother) or two of the X and two of the Y (for a father). Then these four sets are split up, with one complete set of chromosomes being wrapped up in a new nucleus, and split up into four new cells.

    Usually, in a female only one of these will become a new ovum; the others will be what are called "polar bodies". But here's the thing. At the same time a copy of, say, the X chromosome that a woman inherited from her father was recombined with its identical twin, then the copy she received from her mother was also likely recombined with its identical twin. Any of the four new cells might become an ovum, so there are two chances the X in that ovum with be from grandpa only; and two chances it will be from grandma only.

    If you're interested, there's information on the process -- called meiosis -- all over the internet. (Some of it is even correct.) For example, there's https://en.wikipedia.org/wiki/Meiosis.
    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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  15. #18
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    Thanks, all, for your responses.

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  17. #19
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    Quote Originally Posted by Solothurn View Post
    A 1st cousin 1R got a match on FTDNA recently and the X is huge compared to my 1st cousin 1R X comparison.

    Unfortunately both have unknown fathers so we are unable to help one another at this stage.
    Yeah, 55 cM is over a fourth of the entire chromosome. (That's 195.93 cM, as calculated at FTDNA. http://isogg.org/wiki/CentiMorgan)

    Is this 1st cousin once removed perhaps female? [You already said the two have unknown fathers, so I'm assuming the relationship is on their fathers' sides.] In that case both 1st cousin and 1st cousin 1R would be identical* -- as far as just the X chromosome is concerned, that is.

    My largest X-chromosome match with anyone who isn't my daughter or one of my siblings, is with a 3rd cousin. We actually share two segments that are reasonably large: one is 33.54 cM, and the other is 24.49 cM. Together that's actually 58.03 cM.

    However, the 3rd cousin and I are both male, so our matching on the X chromosome may be just as large as the matches would be between our mothers -- who would have been 2nd cousins.

    *Well, not identical. Obviously, the daughter would have two X chromosomes and her father would only have one. What I meant to say was that her paternal X chromosome would be identical to his maternal (and only) X chromosome.
    Last edited by geebee; 02-03-2017 at 05:41 PM.
    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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  19. #20
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    Yes

    This person is related to my cousin 1R but does not share any DNA with myself or my paternal 1st cousin. BTW this match has more on the X than my cousin and our cousin 1R. Actually my dad's cousin!

    FTDNA predict 3rd cousin. The match may well be on their maternal side but the only living relative is not keen on testing

    [QUOTE=geebee;211815]Is this 1st cousin once removed perhaps female? QUOTE]
    AncestryDNA link: Hidden Content

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    My known ancestral lines:

    U152: A9024/BY3644 @ Vallum Aelium
    H1c3b (mother)
    U4a3 (paternal grandfather's mother)
    U198 (paternal gg grandfather)
    K1a-T195C! (paternal grandmother)

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