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. Our jewish DNA line (one grandad) 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 (update these now look to be related via an unknown Norwegian ancestor in Newfoundland long ago).

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:

68 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).”

    • How do you use WikiTree properly. I have looked up names many times and found nothing of ‘current’ interest.

      • You use wikitree by uploading your own gedcom and adding the DNA test information to those relatives who have done it

  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?

    John

  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,
    Tabby

  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

  21. Kitty,
    Can you please help me to understand how the relationships differ from when a person is matching you higher in the X-Dna than the Autosomal vs. higher Autosomal than X-dna?

    For example: EG is matching my brother Autosomal 7.5 7.5 7.5 and X DNA 20.1 20.1, while DB is matching him Autosomal 27.5 17.2 4.5 and X DNA 7.4 7.4

    I don’t understand what the higher value in Autosomal vs. X means in terms of relatives and their relationships.

    Any advice is greatly appreciated!
    Thanks

  22. DNA inheritance can be quite random after close famy so there is no definitive answer to your question.

    Larger chunks tend to be from close relatives, the more the closer, so DB looks to be about a 3rd cousin.

    A male I only gets his X from his mother so look at the maternal line for the common ancestor BUT since X can last much longer due to its inheritance path that 7cM can be from a different ancestor

    EG could also be a 3rd or 4th cousin or related by more than one ancestor

    • Thank you Kitty,

      so I think what you are saying is both people DB and EG are both related to my brother on our mother’s side ( by one of the people on our Xdna pedigree chart). DB is possible 3rd cousin while EG is 3rd or 4th, correct?

      • Susan,
        Not exactly,

        yes to DB although his X could be from a different ancestor so not necessarily your brother’s mother’s side although that is most likely.

        However EG could be a much more distant cousin with two common ancestors, one for the X and one for the autosomal … or a real 4th.
        Autosomal DNA is not cut and dried

  23. Thank you for your informative article. I was just blown away tonight after I uploaded my fraternal twin daughters DNA to Gedmatch. One of my daughters shares absolutely no X with my mother. I am now assuming she inherited the x unbroken from me , from my dad and in turn from his mother, her great grandmother. Is this normal that an unbroken female line from my mother and her granddaughter shares 0 on the X?

  24. Mskitty – can you help me with these numbers

    William is my brother
    Shelby is me
    Grant is a DNA match and X match with my brother William.
    This is how the chart looks – do you think Grant is a traceable relative.
    Is he likely to be connected to our mothers line via his mothers line?

    William & Grant chromosome 15 – Genetic distance 12.9 SNP 2318
    Chromosome X Genetic distance 20.8 SNP 2459

    Shelby & Grant. 1. 26.8 SNP 3122
    4. 16.0 SNP 1386
    15. 15.1 SNP 2655

  25. Hi Shelby
    This sure looks like a relative you could find – 3rd/4th cousin. It is very likely, because of the X match, that you are related on both your mother’s lines, although not certain because another ancestor could have provided that X.
    Does Grant match you and your brother at the same place on chr 15? In other words, triangulating?

  26. I still do not know what an X is supposed to show. What does an X match mean? I see that it goes way back but if it is so far back why even use it? I had my brother and my son tested at Family Tree and still can’t figure if this is a useful tool or not.

    • Focus on the first 22 pairs of chromosomes. They are more useful.
      The X can help with a specific inheritance question or show a match that does not show elsewhere. It may or may not be useful to you.
      Sorry this is just not straightforward.

  27. My husband had the Y DNA test to 37 markers he has had literally thousands match to level 12 even though a lot of these people have tested to the highest markers. Every time testing is done we get more level 12. The only time more than one surname matches is if a family group have tested is this common ?

    • Heather
      This is very common for the R1b haplogroup and perhaps others. I never even look at my R1b Dad’s matches unless they match for at least 37 and have set it so I do not get emails for lower matches. He originally had about 6000 matches at 12 markers.

  28. I know I asked a similar question a couple of years ago but I am still confused about the amount of X DNA I share with my maternal half-aunt.
    We only share 15cM on the X chromosome, the story is she is my grandmother’s daughter but she had a different father than my mother. If we descend from the same female line is it possible to share that small of DNA?
    Or is it more likely she was the product of a family adoption from my great grandfather or one of his brothers? I understand that we wouldn’t share any passed from her father.

  29. Dee, this is perfectly possible although low. The expected amount is about 47cM. If you shared no X at all that would be worrisome.

    A woman gets X from both her parents, that is what makes her female. Your maternal half-aunt will usually share some X with your mother, the amount is whatever their mother gave them that was the same, around 50% (93cM) but it can vary widely. In other words, your maternal grandmother had two X chromosomes which she recombined differently for each daughter. Then the X you got from your mother would be from your mother’s recombination of her two X chromosomes, so about 25% is the likely share with your half maternal aunt, but again, it can vary widely.

  30. Two full siblings, male and female should share some X correct? They share 2,687 cms and I was told they are full siblings.
    Thanks

  31. Hi Kitty,

    I am new to the World of DNA, but very intrigued. I hope you can help me figuring out this X dilemma. A woman has contacted me because she shares chrX 64.1cM and SNP’s 4528 with my sister and I. She was adopted and I can tell from comparisons that she is definitely from my father’s side of the family. How far back should I go to start looking for our MCRA? Many thanks!

    • If she shares X with you, then I assume you are female for it to be from your father”s side? Men only get X from their mothers. That much X is likely to be recent so out to gg- grandparents to start with. Try to get more cousins to test to help narrow this down. U as the adoptee looked at materials from DNAadoption,com ?

      • Yes, I’m a female. I have my (and my siblings) DNA over at Ancestry.com and I’ve uploaded all of our DNA to GEDMatch. This adopted female cousin has her DNA at Ancestry and GEDMatch, as well.

        I haven’t gone over to DNAdoption.com. I’l check that out now.

  32. Though most of the y chromosome doesn’t recombine with the x, there are two or three short pseudoautosomal regions on the x and y chromosomes that do There is a 4th or 5th cousin I match with; on the x chromosome according to gedmatch and familytreedna. We match on ancestryDna, too, but that doesn’t show which chromosomes we match on. We both have two consecutive males who descend from the most common recent ancestors we share. Now if they didn’t recombine at all, neither of the second male of each of ours would have any of that dna at all. Only the first male would have the x dna since the second male would’ve received a y. In my case, it’s my paternal grandfather who would’ve recieved the x from his mother; my dad received a y, of course. I don’t remember exactly which two males in a row were these cousin’s ancestors.. grandfather/father like me or great grandfather/grandfather.. ?
    It threw me off at first seeing the x matching we share until I started to research about recombination of the x and y.
    All in all, the x chromosome is not an exact replica passed down from father to daughter. ONLY if it were that ‘simple’ it would make it a little easier to know which lines we could look at. 🙂

  33. Dear Kitty,

    My mother has the same surname (Fleming) on both sides of her family. Her father was a Fleming and her maternal great grandmother was a Fleming. One of my mother’s female paternal cousins (1st 1R) shares 131 cM of autosomal DNA with my mother, as well as 20.31 cM on the x-chromosome. Does this mean that the two Fleming families are related? If so, how far back might this connection be? Thanks for any insight you may be able to offer.

    Sherry

    P.S.~I don’t know if it matters, but the brother of my mother’s female paternal cousin (1st 1R) above does not share any cM on my mother’s x-chromosome.

  34. Sherry – Your mother got an X from her Dad who got it from his mother. Her female 1st 1R cousin can easily share X from the known shared grandmother unless there are 2 men between the cousin and the shared ancestress. In that case, the 20cM can come from quite a bit further back on a different genealogical pathway (which it can anyway, best to see if it triangulates with another descendant of your great grandmother to confirm its source)

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