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:

44 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


    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.

    • Mindy
      The lack of an X match is not meaningful. In very rare cases male siblings have shared no X

      If it is a close cousin on an expected X inheritance pathway you might try the X one to one on gedmath with lower thresholds, say 400 SNPs and 5cm

  8. Figure this one out!
    My number one, and valid “X” match is a man who is also an autosomal match at about 5.0 degrees of kinship, he also has a maternal line going back to a female that shares the same surname as my most distant female line ancestor, and we can’t figure out who our MRCA might be! Both our Thacker female ancestors are dead-ends with birth locations in either North Carolina or Virginia. They were born in the early 1790s. I also “X” match one of his first cousins, but share no autosomal.

    • Well X matches can be pretty far back in time and it sounds like this one is. So could these females be sisters or cousins? Or their fathers could be cousins or …

  9. Hello, I am fairly new to DNA testing and researching. I am trying to find my mother’s birth family. I have one X match at 105.7 cM actual but this person and I don’t share any autosomal. It seems like a huge number to not consider as relevant in my search. Is this a distant match or something closer, 3-4th cousin? Thank you.

    • The match is likely about a 4th cousin which may or may not be useful in your search. Possibly a 3rd or 3rd 1R.

      2nd and 3rd cousins are very useful and closer relatives are even better … I assume you have looked at the methodology at http://DNAadoption.com ? It works.

      If the match is at GEDmatch try dropping the defaults to about 500 SNPs and 6cM on the one to one autosomal and see if anything turns up.

      • I also have an X match with a male 45.7cM. It shows no autosomal match as well, unless I drop it to really low 500SNP’s and 6cm then it still show’s 0. If I keep dropping it down to say 1cM then it has 66cM total. How close is my relationship to this person?

        • Lee –
          Likely a 4th cousin or further out. You will almost always share autosomal with 3rd cousins and closer, so not likely a 3rd although possible. Such a large X match rates to be a findable relationship.

  10. Oh, thank goodness you are still here! I am attempting to help a young adoptee find her birth father (we are related says 23andMe) I share 2445 SNPs with her on the X, but there is another person she share’s 4356 SNPs with on the X. If what I am reading is correct, that is a large segment and am I correct to pursue his lineage? How far back would you suggest? I am 3-5 generations on him now. Thank you so much!

  11. Yes those are large segments and worth pursuing. How many centimorgans (cMs)? When the X gets passed male to female to male it can go back a long long time so keep going.

    I have a 33cM segment (3326 SNPs) shared on the X with a 4th1R that because of X inheritance patterns we know is 7 generations back, my 5th grandmother’s X.

    The X is not as helpful as closer autosomal matches for helping adoptees. Have you looked at the methodology at DNAadoption.com?

    • Thank you for responding, I have never done anything with adoption before, my past is the traditional “hunt & peck” genealogy. I was hoping to help her using my already extensive FT research and give her at least a starting point. I have read a couple more of your blogs and learned so much already, thank you for all you do!

      • Is there any autosomal matching? How many segments and how large is each one?
        There is no absolute way to determine cousinship from match sizes once you get beyond second cousins due to the randomness of DNA inheritance.
        Because recombination does not happen for the X when it passes through a man it can reach further back in time. If it is an X only match (no autosomal) anything from a 4th to a further distant cousin.
        The chromosome match sizes here are helpful but again X can go further back: http://www.isogg.org/wiki/Autosomal_DNA_statistics

  12. I have a question regarding X matching. Ok, so the way I understand
    it, I won’t inherit anything on my X from my father’s father, so it seems
    I should eliminate the possibility of matching all lines going back from my paternal grandfather. But, what about my grandfather’s sister? Could I match one of her children or grandchildren who test? Or a distant cousin?

    If I want to examine those lines I have to recruit this descendant of the sister to get matches on my paternal grandfather’s side? I think it’s becoming clearer, but I’m still confused. Thanks.

  13. I have a female GEDmatch with Autosomal match, total 95.7, largest 43.4- X-DNA total cM 37.9, largest cM 31.2. What is this relationship?

    Thank you!

  14. I use genome mate for all my matches.
    What is your suggestion for a minimum cM to import for an X match? Different for my mom vs dad when it comes to their individual match?
    How about a minimum when autosomal is also a match or when the X is on its own?


  15. Because women have two X chromosomes, the usual recommendation is 7cM, personally I do not look at less than 10cM. But men have only one X so a male to male 3cM match is good. Male to female, use 7 cM

    • The same but the smaller matches may be too far back in time to find the MRCA so I usually double those numbers to determine which ones to follow up on unless there are common surnames or localities

  16. I am female and have an 82 cM X match (one segment) with a woman adopted in 1955. We share no appreciable autosomal, and she does not match my brother. Questions:
    1. Since she doesn’t match my brother, the common connection is on my maternal side, right?
    2. Isn’t 82 cM X match a rather huge amount?
    3. Can one estimate an MRCA when only X DNA is shared?

    • Right paternal side more likely BUT you need to compare your X to your brother’s to make sure you both got X from your mother in that 82cM region where you match your new DNA cousin. If you do not match your brother for at least 7cM in that same 82cM region, then it could be a match from either side. Also if you are not doing the comparison at GEDmatch, your brother may have too small a match to the new cousin to have it show up as a match for him.

      After 3rd cousins, it is impossible to predict the degree of relationship. One segment matches can go quite far back in time BUT that is a very large match so could be as close as a 3rd/4th cousin

  17. Both parents are deceased, how can I find my dad’s and mother’s side using their female siblings? I have tested two aunts from both side of the side, my mother’s half sister (they did different fathers) and my father’s sister.

    • Renee
      When you have a match, if they also match one of your aunts then you know which side the match is from. If they match neither then you do not know.

  18. Thank you Ms. Kitty. I do match my brother for 33 cM in that region and I am doing the comparison in Gedmatch. I will let you know what I find out.

  19. Good morning Ms. Kitty. My question is this: If you have 2 female first cousins and their fathers are brothers, they will share a high X amount, correct? My mother and her cousin (fathers are brothers) share in the 190 range with a lot of blue on the bottom bar. If I compare to another first cousin match but their X match is low (16cM) with very little blue on the bottom bar, does that mean that their fathers were not brothers but we should be looking at a different relationship with their parents?

    Thank you,

  20. Tabby
    You cannot use the X for that determination as the amount of shared X between brothers is random since they could get very different combinations from their mother. So NO, the daughters can share any amount of X
    If the female cousins had zero shared X then you would look at the autosomal to see if there was a different relationship

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