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X-DNA Advanced Advanced topics about X-Chromosome matching and anthropologic blocks based on SNPs and/or STRs.

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Old 29th May 2017, 08:24 AM
birdie birdie is offline
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Join Date: May 2017
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My father's mother?


Something perplexing is showing in my autosomal results.

I am female and have a match with another female as a 2nd to 4th cousin. I am luckily able to verify, with our family trees, that our most recent common ancestor is indeed a 3rd great-grand-mother of mine. This would be my father's, father's, mother's, father's mother.

I also have an X-Match with this distant female cousin. 3 segments totaling about 15.77 cM, in case that's relevant. It's not much but it's more than my other X-Matches who aren't closer relatives.

The X chromosome my father passed to me strangely seems to come from his father's side of the family, which is Y side, not X.

Am I missing something? This shouldn't happen, correct? If my father's "mother" is not from that side of the family? I have not found any cousins from my father's "mother's" side of the family that match with me knowing, of course, there may not be enough information provided from my other matches. But I do not believe my father's "mother" is related to the side of the family I'm referring to in any way.

My father was color blind, I believe this is a disease of the X chromosome but not sure if that could be related.

Is my father's "mother" not his mother? Can't figure out If I am missing something.

Thanks for any help anyone can provide.
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Old 17th July 2017, 01:15 PM
keigh keigh is offline
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Posts: 477
You got only part of your X chromosome DNA through your father, and he got his solely through his mother. A man doesn't inherit X chromosome DNA from his father. A woman inherits X chromosome from both her father and her mother. So your matches X material may come from the female side of your family.

I know in my own case, I have a number of people who share cousins with me from both sides of my tree. And it does get to be terribly difficult to weed through the generations trying to find which line of my father's side moved out of WV and into my mother's family's areas of TN, TX, MO and such.
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Old 23rd July 2017, 12:17 PM
birdie birdie is offline
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Join Date: May 2017
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Thank you Keigh, for replying.

Well, let me say this. I am 100% sure the distant cousin I am referring to is a relative on my father's side and not my mother's. We both have our family trees publicly available and are able to trace our most recent common ancestor which, for me, is a paternal ancestor. I can trace the X chromosome for this female distant cousin I'm referring to from her tree. She should definitely have this X chromosome. My father's father should have this X chromosome but my father should not (he has passed), and hence, I should not have it either.

I am not knowledgeable enough to know how meaningful three segments of matching DNA from the X chromosome of just over 5 cM's is. Maybe it is true that these matching segments are actually on my maternal X but as mentioned, I am 100% sure I am not related to this distant cousin maternally. It would mean that there are matching segments from my pair of maternal and paternal X chromosomes. If possible, I wonder how probable that is.

Thanks again for your reply.
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Old 23rd July 2017, 02:31 PM
ech124 ech124 is offline
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My understanding is that FTDNA does not use X DNA in calculating relationships. It only displays it to you as additional data.

Do not assume that what is showing on the X DNA is the same relationship that FTDNA has established using the other 22 chromosomes.

This article might help to explain the difficulty involved:

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Old 23rd July 2017, 04:41 PM
birdie birdie is offline
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Join Date: May 2017
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Thanks, ech124 for your reply.

This female distant cousin and I are definitely related. Our most recent common ancestor is a third great-grand-mother to both of us and we do have matching SNP's on chromosomes other than the X chromosome.

I suspect FTDNA reported the X match in light of the fact the matches on the other chromosomes does show we are indeed related. Again, this is verified by comparing our family trees.

These SNP X matches are just over 5cM, not the 7cM mentioned in the article. But there are three matching segments... another reason why I suspect FTDNA determined our relationship as an X match, it makes sense to me in that regard. It's hard to dismiss.

Thank you for the link to the article as well.
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Old 24th July 2017, 03:50 PM
prairielad prairielad is offline
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Join Date: Feb 2011
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All segments which are 1cM/500SNPs or greater are shown when FTDNA matching criteria is met

which is
at least one longest segment of 7cM/500SNPs and 20cM in total DNA shared
one segment of 9cM or greater shared (no total cM minimum)

FTDNA then shows you any segment on X which are 1cM/500SNPs or greater.

But any segment under 7cM is likely to be false positive as each chromosomes comes in pairs (except X in males, they have a Y as the other chr 23 pair) thus each tested position has two values. A matching sequence between two people is just looking for one of the two values to match in each person.
each value can only be one of 4 values, thus the smaller the segment, the higher the chances/odds that it is zig zagging back forth between each persons maternal and paternal values to create a matching sequence.

For X between two females, each has two values (same applies for everyone, male and female on chr 1 thru 22), higher chance that the majority of small segments are false positive

Between a female and male (on X), odds diminish, as comparing females two values to males one.

Between male and male (on X), best odds that smaller segments can be true segments. Comparing only one value to one value.

Just remember, any X match listed may not be a true X match if segment is under 7cM (I would say under 10cM). X matching is only a aid if X segment is of an acceptable size.

I would not discount any other lines on that small of a segment.

Last edited by prairielad; 24th July 2017 at 03:53 PM.
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Old 24th July 2017, 06:17 PM
birdie birdie is offline
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Thanks prairielad for the reply.

What are the odds of having three matching segments, albeit under 7cM, on the X chromosome(s) for two females? The three segments total 15.77 cM.

What is meant by "I would not discount any other lines on that small of a segment."?

Thank you.
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