How to Understand Gedmatch X One-to-One Results

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Have you just started experimenting with the Gedmatch X One-to-One tool?  The results can sometimes be confusing!  What does it mean to share X DNA with someone, and what does it mean when you don’t?  In this post, I’ll help you understand your Gedmatch X One-to-One Results so you can add a full understanding of X DNA to your genealogy toolbox.

You’ll learn:

• How to understand your Gedmatch X One-to-One Results
• What it means to share X DNA with someone
• What it means if you share no X DNA with a DNA match
• How to use X DNA in combination with autosomal DNA results (i.e. “regular” shared DNA information)

How to understand your Gedmatch X One-to-One Results

The Gedmatch X DNA One-to-One comparison results are a little different than the One-to-One autosomal DNA comparison tool results.  They look similar, but shared X DNA is very different than shared autosomal DNA (from your numbered chromosomes).

Below, you can see any example of shared X DNA between me and one of my DNA matches.  Below the image, I will dissect the results and explain all of the aspects of the X DNA comparison results to you:

• Chr:  This is the chromosome that the tool is using for the comparison.  Of course, in this case it is the X chromosome.
• Start Location:  This is the beginning location on the X chromosome where the matching segment begins.  It’s possible to have more than one matching segment, so this information can occasionally be useful (especially for males, who only have one copy of the X chromosome)
• End Location:  This is the end location on the X chromosome where the matching location ends.
• Centimorgans (CM):  This is the size, or genetic length, of the matching segment.  Just as in shared DNA segments on other chromosomes, the length of this segment can provide clues as to how far back the common ancestor might be.  It’s not foolproof, but it’s valuable information.
• SNPs:  This is how many Single Nucleotide Polymorphisms were tested to find this matching segment.  Generally speaking, the higher the number, the better.  A low number of SNPs can mean a false match.

As you might notice, there is no statement estimating generations to common ancestor.  It’s basically impossible to know based solely on X DNA exactly who the DNA match is to me, and so the software doesn’t pretend to be able to predict this at all.

What it means to share X DNA with someone

As you might know, males inherit one copy of X DNA from their mothers and one of their father’s Y chromosome.  Females inherit their father’s entire X chromosome just like it is, and a possibly (or usually) recombined X chromosome from their mother.  When trying to figure out how we are related to an X DNA match, we can use this inheritance pattern to eliminate lines of our family tree that could NOT contain the common ancestor, as well as determine lines that could contain the common ancestor.

We can use outside information contained from family trees posted online or other shared DNA details in order to further concentrate our focus.  You also might be interested in my post “Got Questions About X DNA” to learn a little more about conclusions that you can draw from matching someone on the X chromosome.

What is means if you don’t share X DNA with a match

There are only a few conclusions that you can come to if you share no X DNA with a particular person.  With shared DNA on your numbered chromosomes, if you share none, you can safely assume that you are only very distantly related to a person if you are related at all.  Sharing no X DNA is a little more complicated.  You can still be very closely related to someone and share no DNA on the X chromosome.

Take for example the comparison that I ran between my own Gedmatch kit and my dad’s first cousin.  He is my first cousin once-removed, nephew to my paternal grandmother.  Even though we share about 625 cMs of autosomal DNA on our numbered chromosomes, we share absolutely no X DNA segments.  How can this be?  I did inherit one of my X chromosomes from my dad, who inherited his from his mother.  Since my grandmother and her brother (my DNA match’s father) do share X DNA because they share the same mother.  The problem is that my dad’s first cousin didn’t inherit his X DNA from his father.  Instead, he got it from his mother, and she is not related to me.  The end result is no shared X DNA between us.

Read more in my post “What Does No Shared X DNA Found Mean?”

Is it possible to share no autosomal DNA and still share X DNA?

If you have ever bothered to look at the very end of your Gedmatch One-to-Many results, you might have noticed that there are some people with whom you share no autosomal DNA (from your numbered chromosomes), but you do share some X DNA with them.  This is possible and even common!

For example, my dad has a DNA match with whom he shares about 56 cMs of autosomal DNA and 62.3 cMs of X DNA.  When my dad passed his autosomal DNA down to me, I didn’t get any of the 56 cMs that he shared with his match.  Since he passed down his entire X chromosome to me, I did get the entire 62.3 cMs of X DNA.  That’s a pretty big X match, and we share no autosomal DNA whatsoever.

Conclusion

I hope that this post has helped you understand more about X DNA and your Gedmatch X One-to-One results.  If you have any questions about something that you read in this post, or if you would like to share your own experience with X DNA, please join us in the discussion below.

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