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:

14 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.

  5. I am working on an adoptee tree trying to find an unknown father. I need to decide where to tentively place the 2c1r match and the 3rd c who are not on the X. I am told it is not important if they are not on the X. Why is this not important considering that we are told to make sure someone is not on the X? I have the mother’s side complete. I have filled out an X chart yet but will be soon.

    • If the adoptee is a male then he only gets X from his mother so an X match would likely not be paternal (warning small X matches can be from long ago so possibly a shared ancestor on the other side that is not relevant)

      But a female adoptee gets X from both sides.

      The X is weird and can be misleading since matches there can be so far back in time. I tend to treat it as dubious for recent matching unless there is a LARGE match there, at least 15 to 20 cM …

      • I am not sure I answered your main question.

        Outside of close family, the lack of an X match is not necessarily meaningful. For example, if you have an unrecombined X from your grandfather and your first cousin has inherited an unrecombined X from his wife, you would not have an X match. An unlikely but possible scenario

  6. Hi – my father and a match in GEDMatch have a 39 cm X match. They don’t have much autosomal matching, about 30 cm total with the longest match of around 13 cm. Is there anything I can deduce from this (i.e. how distant of a relation she is), other than that the relation is through my grandmother (dad’s mother)? Also, my father is at least 75% Ashkenazi, and I believe all Ashkenazi on his maternal side. I inherited about 38 cm of that string, and there is another GEDMatch member who matches all of us around 25 cm of the same strand, but I don’t have any info on him/her. Thanks in advance!

  7. Traci –
    Yes on your father’s mother’s side. See Blane Bettinger’s blog for images of X inheritance.
    Ashkenazi is more difficult because distant cousins appear closer. That is a nice sized X match and combined with the autosomal might be a 3rd or 4th cousin possibly once or twice removed if it was not Ashkenazi. Might still be or could be a bit further out.

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