What does shared X DNA really mean?

Since familytreeDNA added X chromosome matching to their family finder, all my favorite bloggers have been writing about how to use it. There is a list of their terrific articles at the end of this post.

Of course those of us who have many family members tested at 23andme have been working with the X chromosome for a while and have some thoughts on how useful it might or might not be.

  1.  Smaller matches on the X will usually be too far back to find the relationship because the X chromosome does not get recombined when a father passes his only X chromosome on to make a daughter. Thus segments can stay together longer in time.
  2. A match on the X can eliminate many ancestors from an autosomal match BUT a small match on the X can be from a different line so this is not guaranteed.
  3. There seems to be less recombination on the X. I have heard of cases where a child got an unrecombined X from their mother. In my own family, the number of recombined segments is quite small. So it seems to me that on the female to female line the X behaves like a sluggish autosome and is not quite as actively recombined as the other chromosomes.

Less recombination examples

Because our maternal line (1/2 Ashkenazi, 1/2 Bavarian Catholic) is from two different population groups, my brother and I can learn which maternal grandparent we got our X DNA from just by looking at the ancestry composition for that chromosome at 23andme:

XchromCompareNotice that my brother got only two segments thus one point of recombination – he has one segment from each maternal grandparent. Whereas my X has two recombination points at each end.

A good blog post about phasing of one’s own X chomosome is at The Lineal Arboretum

X reaches way back in time

My brother and I share 13.9cM (1021 SNPs) on our X with my 100% Ashkenazi husband. We know there is no common ancestry in the last 300 years or so, probably even the last 500. He is of Galician Jewish origin via Vienna. Ours is Southern German Jewish for hundreds of years and we have a good paper trail. However Ashkenazi DNA is difficult to untangle due to many cousin marriages way back when.

Out of the 500 or so shares that my Norwegian descent Dad has in the 23andme database, he has only 6 matches  with greater than 7cM on the X, not including known relatives. We have not found the relationship to any of them and two of them are even Canadian with no known Norse ancestry. We have no known Canadian family members.

Here are some of the better explanations of X inheritance inheritance and helpful charts:

Here are some of the blog posts discussing how to use the new X matching feature at familyTreeDNA:

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8 thoughts on “What does shared X DNA really mean?

  1. I should also have included a link to this good detailed X discussion by David Faux

    http://davidkfaux.org/X_Facts.pdf

    a pertinent quote
    “During each meiosis, the probable cross over (recombination) rate is about one event on the short arm and one to two events on the long arm of the X (or sometimes one event on each arm irrespective of size).”

  2. Very good article and VERY nice to see the best pertinent blogs on the X match all in one place although I would have added Steve Handy’s. Another excellent reference, short sweet, concise, useful, simple and readable is the section on the X match in the FAQ at GedMatch. BTW, the standard “take” on the amount of recombination on the X versus the autosomes is that the X recombines at about 2/3 the rate the autosomes do. I’ll post a link to that if you’d like, but I don’t think an exact figure is needed as long as it is understood that less recombination takes place on the X that on your average autosome.

  3. Hmmm… My mom passed some years ago and we don’t have her DNA. So, if I’m reading this right, would I test my sons… and perhaps the sons of my sisters… to discover what my mother’s X-DNA was?

  4. Not exactly. Your sons have your X.
    You have half your mother’s X recombined. As do any brothers of yours. If you have enough siblings you might be able to reconstruct your mother’s X from all of theirs.

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