An odd X chromosome matching situation

+6 votes
My Dad has auDNA matches on Chr19 with a woman and two of her daughters as follows:

Mother 4,556,239 to 10,489,548, 16.3cM

Daughter1 5,209,490 to 10,620,442, 14.3cM

Daughter2 5,209,490 to 10,524,165, 14.2cM

But when looking at the X chromosome, he has matches with both daughters but not the mother:

Daughter1 27,381,108 to 32,227,629, 8.5cM

Daughter2 27,594,359 to 32,109,730, 7.6cM

Each daughter matches her mother with 2,321 to 154,886,292.

Is the only explanation that the X match is from their father? Or is there some other possibility?

For reference, the kits are A136143 for Dad, A551365 for the mother, and A656169 and A836118 for the daughters.
asked in The Tree House by Chris Hampson G2G6 Mach 8 (89.9k points)

2 Answers

+9 votes
Best answer

Hi, Chris. On the face of it, my guess is that's the only scenario that explains the results: the two daughters' X-chromosome segments that match your dad were inherited from their father. But the X-chromosome has something of a mind of its own, and what we see is not always what we expect.

I started to dive into stuff about the X-chromosome being something of a SNP desert, with relatively few tested SNPs along its length of 156 million base pairs, and the volatility of the pseudo-autosomal regions at the start and end of the chromosome, and what happens if you phase a male's kit at GEDmatch, and yada yada. I'll save it for a blog post. I get gripes about the length of my G2G stuff as it is. But Rob would have beat me to it, anyway.  ;-)

To cut to the chase, here's an important observation from an 18-month-old research paper, "A Genealogical Look at Shared Ancestry on the X Chromosome," from the journal Genetics: "Unfortunately, the X chromosome is short, such that the chance of any signal of recent ancestry on the X decays rather quickly."

There's a lot of information out there that indicates segments of the X can be quite old, that they can pass down for multiple generations--more likely to occur the more males, proportionately, are in that particular line of inheritance--and that a seeming X-chromosome match has the possibility of not aligning with a suspected MRCA at all, but many generations earlier. Conversely, I've never seen any peer-reviewed studies that indicate any degree of assurance of genealogical matching using the X except in a limited fashion. Opinions differ widely.

Roberta Estes wrote just last month, "In general, due to the thin spread of SNPs on the X chromosome, you will need, on average, a cM match that is twice as large as on other chromosomes to be considered of equal weight. In other words, a 10 cM match on the X chromosome would only be genealogically equivalent to approximately a 5 cM match on any other chromosome." And my personal opinion is that the Poisson approximation used in that Genetics paper is likely too generous when modeling possible ancestry; it doesn't adequately take into account that the result of crossing over during meiosis in the X doesn't behave as predictably as do the autosomal chromosomes.

Bottom line, do these results mean that the X-chromosome segment the two daughters' show as a match to your dad mean that the segment was inherited from their father? Yep; almost a certainty.

Does it mean that the daughters' mother and your dad are related? Maybe...but not closely. That 16cM amount, if valid IBD, would probably be around the 5th cousin level, so MRCA around 4g- or 5g-grandparents. But the segment might be IBS and not valid; triangulation work would be required to validate.

Does it mean that the daughters' mother and her husband are related? Maybe, but probably not. I did a check for runs of homozygosity on GEDmatch against the daughters' kits, and there are some possible, tiny half-IBD or half-IBS areas, but none at the 7cM threshold. If there's a relationship from pedigree collapse, it's too far back to be determinable from the current DNA data.

Does it then mean that your dad and the daughters' father are likely to be recent cousins? Nope; doesn't imply that at all. If the father's test results become available, I'll bet that segment in the X is apparent...but it may be just about the only thing that's shared; shared autosomal DNA segments would be needed corroborate. The two men may have a common ancestor, but she may be many generations ago, perhaps before the genealogical timeframe; the segment may be an artifact that's been transmitted down the line for generations. That's why--despite how valuable and useful the X is in DNA analysis for genealogy--none of the major testing companies report matches based solely on the X-chromosome.

I think these words of wisdom from Jared Smith are a good go-by for dealing with the X-chromosome:

  • "Use the amount of autosomal DNA match to determine relatedness, and X-match data to narrow down to particular family lines."
  • "X-matches that do not share measurable autosomal DNA would generally be distantly related."
answered by Edison Williams G2G6 Pilot (178k points)
selected by Chris Hampson
I knew some smart young man would chime in, and you fit the bill ;)

That's an interesting quote from Roberta, I hadn't seen that before. In other areas people use Poisson because it makes the mathematics easier, that may be the case here. And you are certainly correct about about the X not behaving as we think it should. Roberta wrote about that aspect also.

There are a bunch of common matches with other people on that Chr19 segment, but no genealogical links have yet been found. All of the matches are in the US, Dad's family is exclusively (at least as far as we know of) English. Given the difficulty in tracing immigrants back to their place of origin we may never make the connection.

Thanks for the best answer star, Chris. And even more gratitude for calling me a "young man."  ;-)

Trivia of the day, or xDNA things that make you go, "Hmm..." Sounds like you might appreciate this:

The number of ancestors contributing an X-chromosome at each generation follows the Fibonacci sequence, i.e., 1, 2, 3, 5, 8, 13, 21, 34, and so on: xn = xn-1 + xn-2. Males start with one X-chromosome contributor and females with two, then proceed with the sequence from there. The shapes of hurricanes and even spiral galaxies also follow the Fibonacci sequence.

Edison, you can send me an email with the long explanation any time.  

Where is your blog?

Thanks, Laura! You're officially on my holiday card list.  :-)

Since the new-ish blog is non-commercial and contains no advertising, I don't feel unethical mentioning it:

Thanks Edison!  As an Adoption Angel I am always looking for more ways to help find birth families.  I love your detailed explanations!  I am bookmarking your blog!
Bookmarked also!  And subscribed!
I also subscribed!
+7 votes
I double checked your figures and results, thinking there had to be a mistake somewhere, but no, everything checks out perfectly.  So yes, I think your explanation is the only one possible, that your Dad has 2 different DNA connections to the daughters.  There must be a connection (4th to 6th cousin?) through this mother on chromosome 19, and there is a different connection through his mother on the X chromosome to the mother of the father of these daughters.  I can't think of any other way it could happen.

That's actually a really interesting situation.  Both may be quite distant, the X connection may be closer.
answered by Rob Jacobson G2G6 Pilot (103k points)
Thanks for checking, Rob. I was beginning to doubt my own competence.

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