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Thread: 23andme chromosome (graphic) match as an indicator of ancestry.

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    23andme chromosome (graphic) match as an indicator of ancestry.

    I sent a DNA sample to both Ancestry and 23andme. While I spend most of the time is on ancestry, I spent hours (too many) looking at 4th cousins on 23andme. As you may know, there is a "view DNA details" button when looking at relatives on 23andme.
    My question is this : A lot of my 4th (and 3rd) cousins were a match on the "X chromosome" (yeah the 23rd one). It was a really large (graphically speaking) match. Maybe 30% of the X chromosome and always in the upper part of the graphic. I always assumed this was a good indicator of family relationships. So I found 14 cousins, mostly 3rd and 4th cousins (same 2nd and 3rd Great Grandparents) that matched this criterion. None of them were on my ancestry and I have over 1500 relatives there. Does this kind of evidence from 23andme actually point to a relative ?? DNA evidence is helpful when an ancestor had an affair and the paramour's family keeps popping up without explanation (yeah, had that too). So. before I go digging around and asking uncomfortable questions, I want to be reassured there is a real family connection based on 23andme.
    Much thanks, in advance, for your input.
    Jim Perdue

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    Quote Originally Posted by Vivere View Post
    I sent a DNA sample to both Ancestry and 23andme. While I spend most of the time is on ancestry, I spent hours (too many) looking at 4th cousins on 23andme. As you may know, there is a "view DNA details" button when looking at relatives on 23andme.
    My question is this : A lot of my 4th (and 3rd) cousins were a match on the "X chromosome" (yeah the 23rd one). It was a really large (graphically speaking) match. Maybe 30% of the X chromosome and always in the upper part of the graphic. I always assumed this was a good indicator of family relationships. So I found 14 cousins, mostly 3rd and 4th cousins (same 2nd and 3rd Great Grandparents) that matched this criterion. None of them were on my ancestry and I have over 1500 relatives there. Does this kind of evidence from 23andme actually point to a relative ?? DNA evidence is helpful when an ancestor had an affair and the paramour's family keeps popping up without explanation (yeah, had that too). So. before I go digging around and asking uncomfortable questions, I want to be reassured there is a real family connection based on 23andme.
    Much thanks, in advance, for your input.
    Jim Perdue
    Yes, I'd say a match on the X chromosome that covers as much as 30% of the X chromosome is "a good indicator of family relationships". It does at least indicate that this match is related to you on your mother's side -- although it doesn't necessarily preclude shared segments on the autosomes being from your father's side. This would be especially true if they happen to be related to each other, but can sometimes be true even when they aren't.

    To illustrate what I mean, consider my 2nd great grandparents "George and Clarissa". George was a double brother-in-law to Clarissa's brother "Sanford", and Clarissa was a double sister-in-law to George's sister "Elizabeth". Why? Because Sanford married Elizabeth. So George was Sanford's brother-in-law both because Sanford was his wife's brother, but also because Sanford was George's sister's husband. Likewise Clarissa was Elizabeth's sister-in-law both because Elizabeth was her husband's sister and because Elizabeth was Clarissa's brother's wife.

    Obviously, the children of George and Clarissa were related to the children of Sanford and Elizabeth on both sides -- yet neither husband and wife pair were related to each other.

    As far as none of the 23andMe relatives being at Ancestry, my experience is that most people who have tested at one company haven't also tested at the other. There are exceptions, of course. For example, I've tested at both. So have a few of my relatives. But not most of them.

    Those that have, however, are very helpful. I can compare our shared segments at 23andMe, which is great. But if they have tested at Ancestry, I'm usually able to find a lot more folks in our shared match list -- even though I'm unable to see any actual shared segments. Sometimes, I'm unable to find the same person at both companies, but I can see close relatives who each tested at a different company.

    For example, I have a 2nd cousin once removed who tested at Ancestry. Her daughter tested at 23andMe, and it's likely that all the segments I share with the daughter came from the mother -- since I don't seem to be related to the father. I also have two 2nd cousins who are 1st cousins to each other, one of whom tested at 23andMe and the other of whom tested at Ancestry. In this case, I lucked out even more in that the one who tested at Ancestry uploaded his DNA file to GEDmatch.

    This brings me back to the subject of DNA. It turns out that while I don't share any segments on the X chromosome with my 2nd cousin at 23andMe, I share two segments on the X chromosome with my 2nd cousin at Ancestry. One of these is about 15 cM, while the other is 27 cM. In addition, I share a 31 cM segment on the X chromosome with a 2nd cousin once removed at 23andMe. This segment has a fair amount of overlap with one of the segments I share with my 2nd cousin at Ancestry. My 2nd cousin once removed is a 1st cousin once removed to both my 2nd cousins.

    The reason I share no X chromosome DNA with one of these three but I do share some with the other two lies in how we're descended from our common great grandparents (2nd great grandparents of the 2nd cousin once removed). All three of these relatives are descended from one of my grandfather's brothers. His name was Marion. One of the 2nd cousins is descended from Marion's son, and even though Marion did inherit an X chromosome from his mother -- my great grandmother -- Marion did not pass that X chromosome on to his son.

    My 2nd cousin at Ancestry, however, is descended from one of Marion's daughters. She not only inherited a copy of her father's X chromosome, she passed on at least a portion of that X chromosome to her own son -- my 2nd cousin. Likewise, my 2nd cousin once removed at 23andMe is also descended from one of Marion's daughters. This daughter was my 2nd cousin once removed's maternal grandmother. She, too, inherited a copy of Marion's X chromosome, and passed on at least a portion of it to her daughter, who passed on at least a portion to her son.

    On my side, my grandfather inherited an X chromosome from his mother -- who was the same woman as Marion's mother. However, the two brother's probably did not inherit identical X chromosomes, even though there was obviously some overlap. My grandfather then passed on a copy of this X chromosome to my mother, who passed an unrecombined copy of this chromosome to two of my siblings -- one of my brothers, and one of my sisters. All of the matches the two of them have on the X chromosome trace back to our maternal grandfather, and collectively these matches span the full chromosome.

    For my part, I inherited almost all of this same chromosome. But I have about 20 cM surrounding the centromere of my X chromosome which came from the opposite grandparent -- my maternal grandmother. Apart from matches within this region, all of my X chromosome matches are to relatives of my maternal grandfather.

    So to get back to to your question: Yes, it is highly likely that these matches represent real relatives. Determining exactly what degree is a more difficult matter, particularly if any of these matches include only a segment on the X chromosome. These segments can sometimes persist for a fairly large number of generations. As illustration, I share a 24 cM segment and a 34 cM segment on the X chromosome with someone who is no more closely related to me 3rd cousin twice removed.

    The thing is, though, this cousin is actually related to me in at least four additional ways; and through both of his parents, who were 1st cousins. That means there are lots of ways we could have ended up with shared segments on our autosomes. However, the number of ways we could have ended up with shared segments on our X chromosomes are far fewer.

    In the first place, since we're both males, these segments had to come from our mothers. In my case, however, I know that the segments came from my mother's father -- and therefore from his mother. I was fortunate in that both my cousin and I have fairly complete trees back to 4th or 5th great grandparents, and sometimes beyond. But, by looking for a point of intersection between possible X-chromosome pathways on each side, I was able to determine that my 4th great grandmother Rachael Jones -- wife of Henry "Hunting Shirt" Sizemore -- was the likely source of both of these segments.

    Rachael is both my cousin's 3rd great grandmother and his 2nd great grandmother. Only one of these relationships, however, lies in an "X-path" from my cousin -- though our shared autosomal segments might have been passed along either path or even be from an entirely different shared ancestor (or ancestors).
    Last edited by geebee; 01-24-2020 at 04:33 AM.
    Besides British-German-Catalan, ancestry includes smaller amounts of French, Irish, Swiss, Choctaw & another NA tribe, possibly Catawba. Avatar picture is: my father, his father, & his father's father; baby is my eldest brother.

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    Where can I find this 'View DNA details ' button?
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    Quote Originally Posted by firemonkey View Post
    Where can I find this 'View DNA details ' button?
    In DNA Relatives, when you click on the name of a relative you'll see a profile page for that person. The button is in the section entitled "Your genetic relationship", below the graphic of your "predicted relationship". However, for it to show you anything but a percentage, you either have to be sharing with this person or they have to be a "public" match.

    If either of these conditions apply, then clicking "View DNA Details" will reveal your cM shared and will display a small graphic showing the placement of your shared segments. It won't give you the length of each segment. For that you need a feature that allows you to "Compare your DNA with close and distant relatives".

    Besides British-German-Catalan, ancestry includes smaller amounts of French, Irish, Swiss, Choctaw & another NA tribe, possibly Catawba. Avatar picture is: my father, his father, & his father's father; baby is my eldest brother.

    GB

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    Thank you for your replies. I appreciate the chance to learn more about both ancestry and DNA. I forgot to add something in my original post, and that is the 3rd cousin in question and I have 47.41 cM and 4004 SNPs that are an identity of half. This is probably much more helpful to the experienced person than my naive expression of "30%". There are over a dozen DNA matches with similar cM and SNP correlation on the X chromosome. Since none of the names are on my ancestry tree I was searching both my Mother and Father side of the family, at least now I know to begin with my Mother's side.
    Your educated and knowledgeable replies are greatly appreciated.
    Jim Perdue

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