Is it unusual to share 115 cM of X-DNA with a 440 cM AU DNA match?

+3 votes
I have a 440 au DNA match, with whom I share 115 cM of X-DNA, according to GEDMatch. She matches all my known maternal DNA matches, including an X-DNA match of 45.4 cM with my first cousin once removed (409 cM au with me), with whom she shares 60 cM of au DNA. All these matches are from Ancestry.

Her mother is adopted.

Do any of y'all have any suggestions as to how I might narrow down where the X-DNA might be coming from? Her mother passed, so she is not a source of information. I am aware of the inheritance pattern of X-DNA in males and females.

Thanks for any information you might be able to provide.
in The Tree House by D Kenney G2G6 (10.0k points)
I am SO sorry, y'all. I left out what might be a crucial piece of information: I am also adopted, BUT I have figured out both sides of the family tree and they have been confirmed by the cousins on both sides. My apologies for the oversight.

3 Answers

+3 votes

Your DNA Ancestors page in Family Tree and Tools shows who you could have inherited X-DNA from. Scroll down to the X Chromosome section.

by Ellen Smith G2G Astronaut (1.3m points)
+5 votes
It is hard for me to piece this together without concluding that you and your match are half-first cousins, each sharing your maternal grandfather.  (There is an outside chance that you could be full first cousins, but that is a very low probability if you share 440cM.)

The aU number suggests that the match is around 1C1R or equivalent, i.e., a half-first cousin., or maybe a 2C. But I see no way to get you that X-match number at second cousin level except assuming that multiple females had children with no crossover on the X-chromosome during meiosis. I don't know how common it is to have no X-crossover, so if it is more common than I think then this analysis will be incomplete.

Anyhow, if grandpa had two daughters, he would have given his entire X-chromosome to each of them.

If each of those daughters has a child, the X-chromosome she gives that child is recombined, so is half her paternal X-chromosome and half her maternal chromosome. So, the children of these two sisters are expected to share only 1/4 of their X-chromosomes, i.e., 25%. That's already too small for your match.

Because you believe that you and your match both got this X-chromosome through your mothers, both would have the chance to recombine. So you can't insert a male in the possible connections and still reasonably expect to share fully half of an X-chromosome.

So I guess the question is -- do you know *anything* about your match's mother's adoption circumstances? Was your maternal grandfather alive and an adult at that time, and was she in the general vicinity where your maternal grandfather might have been?

The other question is -- is it possible to test your mom and/or hers? That would give further clarity.
by Barry Smith G2G6 Pilot (237k points)
Paternal link is most likely. As daughters inherit 100% of the father's X chromosome as he has only one to give! i.e. 1/2 1C though two half sisters with the same father and different mothers. Both half sisters would have an identical paternal X chromosome and could theoretically pass it whole, without much or any recombination, to their children.
Yup, it seems we agree. I Googled the frequency that no crossover happens on the X, but nothing showed up.
Barry, thank you for your thoughtful reply. Please see my above comment about my negligence in mentioning that I am also adopted. I have figured out both sides of my family tree, and they have been confirmed by cousins on both sides. My sincerest apologies. I was trying so hard to explain the numbers and the DNA relationships between this match and this maternal first cousin and myself and their sources that I failed to include that detail.

Both our mothers are deceased, so no further testing at that level. Mine was born in 1927, hers in 1950. I have another first cousin once removed and a full first cousin on the maternal side (their mother) who may be willing to upload to GedMatch. My maternal grandfather passed in 1965, but he was in Arkansas, not Michigan. The two sides were never in the same vicinity until the woman who gave birth to me and her two sisters moved from Arkansas to Michigan in the 1940s.

Given what you have explained, I now wonder whether there is some inheritance on the paternal side? The 115 cM X-match's mother was born in Michigan, where my paternal DNA donor was also born and raised. His mother was born there in 1910, so I'm wondering if there is crossover from my grandmother's line?

Would encouraging my female first cousin on the paternal side to upload to GEDMatch help to determine this? I don't know whether she still has access to her Ancestry DNA account, so I'm not sure whether she could get her hands on her Raw DNA file.

Many thanks for your input.

But it is still true that "She matches all my known maternal DNA matches"? You are certain these maternal matches are on that side? That this woman is on your maternal side. If you have some unusual endogamy, you could be related on both your paternal and maternal aides. But otherwise, if you know the match is on your maternal side, then what I wrote still stands.

If your cousin has tested, then yes, getting her tested on the same site as this mystery match’s would be useful. It doesn’t have to be GEDmatch, although that is probably the best. They should still be close enough to your match that if they don’t match, you can be assured the match is on your other side.

+2 votes
440cM would mean that she is only 5 steps from you.
If that half match was your maternal grandfather.
The auDNA chain would be, your mother (3400cM) - maternal grandfather (1700cM) - her mother (850cM) - your match (425cM)... and the X DNA from your maternal grandfather would be:
M-Grandfather to your mother 100% of his X-DNA
Your mother to you 0-100% of your M-GFs X-DNA
M-Grandfather to her mother, same 100% of his X-DNA
Her mother to your match 0-100% of your M-GFs X-DNA.
That is by far the most likely scenario with such a big X-DNA match. with the link being your maternal grandfather... and your mother's being half siblings.
The likelihood that you would share such a big X-DNA.
If your match was through an unknown full sibling of a grandparent, it is very unlikely (if not impossible) that you would share such a big X-DNA segment.
Kind regards

Siggi Jónas
by Sigurður Eysteinsson G2G2 (2.6k points)
edited by Sigurður Eysteinsson
Thank you for your reply.

So if my maternal grandfather had an additional daughter which I did not know about--a half-sibling to the woman who gave birth to me, and this maternal half-aunt was the mother of this 115 cM X-DNA match, this would explain the size of the X-DNA match? Intriguing. All three of his daughters are deceased, but THEIR daughters are not. I'm going to inquire as to what they might know, if anything.

Thank you for your insight.

Yes, that is by far the most likely possibility... if not the only... given the size of your X-match.
The full list of possible auDNA connections with your 440cM match, according to DNApainter's shared cM tools, are:
(I put the most likely ones in bold based on the assumption that there is not a great generational difference between you and your match).

83% chance: Great-Great-Aunt / Uncle, Half Great-Aunt / Uncle, Half 1C, 1C1R, Half Great-Niece / Nephew, Great-Great-Niece / Nephew

16% chance: 2C, Half 1C1R, 1C2R

1% chance: Great-Grandparent †, Great-Grandchild †, Half Aunt / Uncle †, Half Niece / Nephew †, Great-Aunt / Uncle 1C, Great-Niece / Nephew

†=outside 99 percentile range. i.e. very very unlikely... if not an error.

So, there are other auDNA match possibilities, leaving the large X-match aside.

All the best
Siggi Jónas


Thank you!

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