What's hidden in your chromosomes?

DNA & Genealogy

X-DNA's helpful inheritance patterns

When you test your autosomal DNA, your X-DNA is also examined and included in your raw data &/or match results.  When you match someone on your autosomal DNA and also share segments on the X-chromosome, the unique inheritance pattern of X-chromosomes can help isolate your common ancestor to particular lines of your ancestry.

 

X-Matches in autosomal results

Family Tree DNA's Family Finder test identifies X-Matches with people you already match via your autosomal DNA, and clearly displays them in your match list (as below).

You can also view the X-match detail in the FTDNA chromosome browser, and download all chromosome browser data to Excel or other software or sites for further analysis if desired (eg. GEDmatch).

 

X matches on FTDNA's Family Finder

 

23andMe X-DNA can be viewed in the 23andMe chromosome browser or downloaded, but only for those matches who choose to share genomes with you.  The raw data can be transferred to GEDmatch for comparison with other kits in the GEDmatch database.

AncestryDNA does not provide or show any X-DNA information, so X-chromosome information can only be accessed by transferring a copy of the raw data to FTDNA or GEDmatch for analysis.

 

X-Chromosomes

Your sex is determined by the X and Y sex chromosomes: chromosome pair number 23.

Males inherit a Y chromosome from their fathers and an X chromosome from their mothers.  So all their X-DNA is inherited from their mother.

Females inherit an X chromosome from their fathers, and an X chromosome from their mothers.

 

X Chromosome Inheritance

(Click on image above to open in a new tab, then right-click to save a copy if desired)

 

A mother contributes an X chromosome that is usually a recombined mix of both of her X chromosomes (but not always), and fathers contribute their whole X chromosome intact to their daughters.

 

Because males only inherit an X chromosome from their mothers, if a male has an X-match in his DNA results, the shared ancestor must be on an ancestral line that follows the male X-inheritance pattern, as below (in green):

 

X-chromosome Inheritance Chart - Male

 (Click on chart above to open a higher resolution image in a new tab, then right-click to save a copy if desired)

 

Because females inherit X chromosomes from both parents, if a female has an X-match in her DNA results, the shared ancestor must be on an ancestral line that follows the female X-inheritance pattern, as below (in green):

 

X-chromosome Inheritance Chart - Female

(Click on chart above to open a higher resolution image in a new tab, then right-click to save a copy if desired)

 

X-DNA cannot be passed down through two successive male generations.

 

Because some X chromosomes pass down intact (through males) and skip a generation without recombining, and others are recombined (through females), the average expected percentage of shared X-DNA at each generation varies depending on the branch of the pedigree chart.

 

X-chromosome percentage inheritance chart - Male

(Click on chart above to open a higher resolution image in a new tab, then right-click to save a copy if desired)

 

Compare the percentages across the same generations below.

For example, at the great-great-great-grandparent level, notice how the expected percentage of inherited X-DNA is only 3.1% on the far right direct maternal line, versus 12.5% on the left-most paternal line (and some other lines) at the same generation level.

 

X-chromosome percentage inheritance chart - Female

(Click on chart above to open a higher resolution image in a new tab, then right-click to save a copy if desired) 

 

With no X inherited from some ancestors, varying amounts inherited from others, the randomness of DNA recombination at each generation, and occasional sticky segments passed down intact over several generations, X-DNA can be quite unpredictable and difficult to interpret exactly where it came from.

It is common to share segments of X-DNA with people who share no significant amount of autosomal DNA.

Males generally get far fewer X-matches than females.

X-DNA's best and most practical use is for isolating matches to particular family lines, even though the amount inherited cannot tell you from whom or how far back it came.

Nor can the absence of any X-DNA disprove your relationship (except for immediate family members - see below).

Focus on larger X-matches, such as 20cM or more, as smaller segments may not be reliable.  For FTDNA X-matches, use the chromosome browser to check the segment sizes.

 

Download link for Excel spreadsheet for X inheritance pattern male & female surnamesExcel Surname Template

Click the image to the right to download an Excel spreadsheet surname template to create your own X inheritance pattern pedigree charts - create one for each person that you test (separate sheets for male & female charts).

 

 

 

Practical uses of X-DNA

  • If a male shares X-DNA with a match, then the ancestor in common will be on his mother's ancestral lines, according to the X inheritance patterns in the 'Male' charts above.
  • If siblings have tested their autosomal DNA, and a brother has X-matches in common with his sister(s), then the sisters will know that those particular X-matches must have come from their mother, as their brother could only have inherited them from their mother
  • If brothers share very little X-DNA with each other, one would have inherited most of his X from his mother's father and the other would have inherited most of his X from his mother's mother, or they inherited the exact opposite of a recombined X.
  • If brothers share most of their X-DNA, they would have inherited it either from the same maternal grandparent, or the same or very similar recombined X from both maternal grandparents.
  • If half sisters with the same mother share very little X-DNA with each other, one would have inherited most of her X from her mother's father and the other would have inherited most of her X from her mother's mother, or they each inherited the exact opposite of a recombined X.
  • If half sisters with the same mother share most of their X-DNA, they would have inherited it either from the same maternal grandparent, or the same or very similar recombined X.
  • A daughter will share a whole X chromosome with her father.
  • Full sisters will share a whole X chromosome, from their father.
  • Half sisters will share a whole X chromosome if they have the same father.
  • You can attribute X segments to particular grandparents by comparing your X-DNA with cousins and other close relatives from each side of your family.
  • Don't forget to ask your match to look at the charts above and work out which of their lines you could match them on, to narrow down the possible branches from both sides of the family!

 

Useful Links

 

 

Link to Facebook Group - Using DNA for Genealogy - Australia & NZ

Facebook group - DNA for Genealogy UK

 

 

23 Pairs of Chromosomes. One Unique You. Get your DNA story at 23andMe.com.

 

Overall Rating (5)

5 out of 5 stars
Load Previous Comments
  • Tonia Bootle

    Rated 5 out of 5 stars

    Hi Louise,
    Thanks for this article, it was so easy to understand. I'm new to this, and recently had my brother tested also through 23andme, as I'm trying to find out who my biological grandfather was. I ran his raw data through GEDMATCH, hoping to identify a few X matches (which would only be mum's side), and when I ran the 'one to many' report, there was only 1 match for his X data - out of 2000 matches! Can this be accurate? I knew this process would be similar to finding a needle in a haystack, but only 1 in 2000? Is there something I'm missing here? Is this another area of his data I should be focusing on or drilling down on through another report or tool? I'm so excited to finally be able to start researching, but also overwhelmed, so any advice you could give me here would be greatly appreciated! :)

  • Hi Tonia, Thank you for your message. Before running the 'One-to-many matches' report on Gedmatch, did you select the ‘X’ version of the report, or the regular ‘Autosomal’ report and search or sort by the ‘X’ column? Males generally get very few X-DNA matches, but the specific X version of the 'One-to-many matches' report usually finds a lot more, even if they share no or very little autosomal DNA, as they are often hidden below the 2000 cutoff of the autosomal report. I hope that helps? Good luck! Louise :)

  • Nancy Kingston

    Rated 5 out of 5 stars

    Dear Louise,
    I'm a 76-year old adoptee. I was born in and live in southern California. I connected with my Scot-Irish birth mother later in her life and she died never revealing the identity of my birth father or any information about him. Several years after her birth, I had autosomal DNA testing and I learned that my birth father is predominately 47% Finnish with a slight amount of Volga-Ural Russian. Also, my mother's brother agreed to have his DNA tested to distinguish my maternal and paternal DNA matches. I have around 1,500 5th or greater Finnish DNA cousins residing in Finland and around 60 3rd-4th DNA cousins, also residing in Finland. Due to endogamy, any one of my DNA cousins residing in Finland can share DNA with 500 or more of my Finnish DNA cousins. Since my Finnish grandparents were recent immigrants to the United States, probably 1890 - 1910, I have very few Finnish DNA cousins residing in the United States and none are aware of my birth father.
    My question is regarding the X chromosome that I received from my birth father. I have one female Finnish 3.9 DNA cousin the same age as me residing in Finland with whom I share 62.1 total cM, with the largest of 21.2 cM, and 31.3 total X-DNA cM, with largest of 15.8 cM. This is the largest amount of X-DNA that I share with any of my Finnish cousin DNA matches. Is it worth pursuing how we might be related and, if so, how do I trace her ancestry to a common ancestor through the X-DNA?
    Thank you.

  • John Harrison

    Hi Louise, thank you for leaving this information on the internet. It was a big help.

  • Wayne

    Rated 5 out of 5 stars

    Hi Louise,

    I am lost with my x chromosome connections. My mother and I have both tested atDNA with AncesteryDNA and uploaded to FTDNA and we both share x chromosome (3384 c/morgans). We have been contacted by new found DNA relations whom are a brother and sister. My mother and I share an x chromosome with this sister (mum 448 c/m and myself 346 c/m) but no x with this brother. Which branch do I follow to find how we are related to this brother & sister? Also how close are they to my mother?
    Thanks in advance,
    Wayne

  • Hi Wayne, You get your X-DNA from your mother, and she can get it from either of her parents. The lack of X-match to the brother cannot rule out either of his parents (his sister could get X from either parent) so check the X-inheritance path of his sister and look for a connection (eg. time/location) to ancestors on the X-inheritance path of your mother. A 448cM match to your mother could potentially be her half 1st cousin, a 1st cousin once removed, a half niece/nephew/aunt/uncle, a half great niece/nephew/aunt/uncle or a great grand niece/nephew/aunt/uncle - so consider the matches' ages/generations when working out the connection. Good luck - I hope you can work it out! The relationship calculator at this link is very helpful (just enter the cM): https://dnapainter.com/tools/sharedcm/

  • Susan O.

    Hi Louise,

    I have a peculiar situation and was curious on your insights into the matter. I am a female and my male cousin is coming back as an X match to me. His father was my father's brother. So the reasoning goes, I got some X DNA from my father, but my male cousin would have gotten his X DNA from his mother, who I am unrelated to. How is it possible that we are coming back as an X match? His X DNA, coming from his mother, does have roots in Ireland, but different counties than where his paternal line comes from (not even close in terms of regions in Ireland). I do have in my paternal X DNA an ancestor from an unknown part of Ireland, but my speculation on this ancestor points to a different county in Ireland. Is it possible that there is a common ancestor far back in Ireland, or could this be a false match of sorts?

  • Louise Coakley

    Hi Susan,
    The first thing that you should check is the size of the X-match, as if the tests were done at FTDNA, their total includes the very tiny segments for the X, so you really only want to look at large X-matches, say 20cM or more (ignore any tiny segments). To see the segment sizes, click to the left of the tester's name in the match list, and click on Chromosome Browser to see the actual shared segments (chr 23 is the X chromosome). You can hover on a segment to see its size, or you can click on the 'View this data in a table' link at the top of the chromosome browser. If the segment is quite large, say 20cM or more, then male-female cousins from two brothers must share a different ancestor, as you are correct - they cannot inherit the same X (as the male only gets his X from his mother). You can also easily compare the X segments at Gedmatch, if both you and your cousin have uploaded your data.
    Good luck! Cheers, Louise

  • Kelly

    Rated 5 out of 5 stars

    Hi Louise, thank you for posting this clear and concise explanation! Your Excel tool is quite helpful for visualizing and naming the connections.

    In another comment, you mention 20 cM as a reliable number for confirming a match, but on the other chromosomes, most people treat 7 cM is a good minimum value. Is there a difference with respect to the X chromosome? Is that because it doesn't change much when passed from father to daughter?

    I ask because I have an overlap - a known maternal-maternal cousin (German descent), with a 20.6 cM match, overlaps a known maternal-paternal cousin (Scots-Irish descent) with an 11.8 cM match. I am female, so my assumption is that one of these matches has to be from my dad's side. Is the 11.8 cM match considered unreliable?

    I have other evidence that suggests my parents have shared ancestors going back to Ireland or Scotland, at least 200 years ago. GEDmatch says my parents are unrelated, but the connection may be too old. I was wondering if this is further evidence to illustrate that point, or just noise.

    Thanks again!

  • Louise Coakley

    Hi Kelly,
    Thank you for your comment and feedback. A 7cM minimum threshold is normally recommended for chromosomes 1-22, but a higher minimum (eg. 20cM) is often recommended for chromosome 23 (X), as the SNP density on the X chromosome is less (ie. far fewer positions are examined along the length of the chromosome, so chances of false matches are higher). And yes, the fact that an X passed from father to daughter to child means it can skip a generation without being recombined means that it could be from further back. Your shared paternal-maternal ancestry from generations ago could explain the overlap on the same position of the X-chromosome from different sides of your family. I wouldn't dismiss your matching 11.8cM segment altogether, but use it as evidence for now, and continue to look for more evidence over time. Perhaps you will eventually find a match who triangulates on that same segment, and it will help. Also consider testing other relatives, as a sibling or cousin could share more X with this match; or the match's relatives may share more X with you. It is difficult to make assumptions about the X, as it can happen that two siblings might not share any X-DNA at all, and yet cousins from many generations ago on certain inheritance paths can share quite large segments! It sounds as though you have grasped the concept well, which is great! Stay open-minded, and keep looking for more evidence! :) Cheers, Louise

Add comment