X DNA Match But No Autosomal?

What does it mean if you have an X DNA match, but share no autosomal genes? In this post, find out whether these are true genetic relatives, and how to determine your relationship.

It can be strange to have a DNA match that shares an X-DNA segment, but absolutely no autosomal DNA segments. If you add in the perplexing way the way that X-DNA is inherited, it is extra confusing.

Are these real matches? If so, how far back would we have to look to find the common ancestor?

These are excellent questions, of course, but you might first wonder how people can even find these types of DNA matches. We don’t see X-DNA only matches on Ancestry because we don’t have X-DNA matches on that site, and 23andMe only shows us our 1500 or so closest genetic matches.

How to find X-DNA matches that don’t share autosomal DNA with you

Most of the DNA testing companies don’t display X-DNA matches if there is no autosomal DNA shared because of minimum thresholds for reporting DNA matches. So, many people resort to other methods to find genetic relatives related only through only X-DNA.

The easiest way to find X-DNA matches is on a website named Gedmatch.

On the Gedmatch One-to-Many Comparison Tool results, you can filter the results to show your top 3000 X-DNA matches. Many of these matches will share no autosomal DNA with you.

When I recently ran this tool in preparation for this article, I discovered that I have at least 3,000 matches on Gedmatch that share only X-DNA with me. Are all of these individuals really related to me?

Are X-DNA matches with no autosomal shared real matches?

It is possible to have many relatives within whom you share X-DNA and no autosomal DNA. We will typically only become aware of these relatives when we find them through DNA testing.

This means that DNA matches that share only X-DNA can be real matches, and indeed related in some way through a shared ancestor. However, it is important to note that X-DNA segments should also be subject to the same scrutiny that we apply to autosomal DNA segments.

In other words, smaller shared X-DNA segments are more likely to be false, and larger shared X-DNA segments are more likely to indicate a more recent common ancestor. However, there are a few extra tidbits of information we should also take into consideration as we consider shared X-DNA segments.

Because of the unique inheritance pattern of X-DNA, segments are not broken up as quickly throughout the generations as they are with autosomal DNA. Many experts suggest that we should use a larger threshold of segment size for X-DNA segments than we would for autosomal DNA to reduce time spent on segments that might from a shared ancestor too far back to identify correctly.

In addition, the nature of X-DNA testing leads to a higher probability of very small segments being false, or identical-by-state, and not inherited from a shared ancestor. These false segments are only coincidentally identical.

We know that most autosomal DNA segments under the size of about 6 cMs are false segments (i.e. identical by state). When these very small segments are legitimate, they can be inherited from a common ancestor 8-10 generations back in a family tree, or more.

How far back to find common ancestor for X-DNA matches when no autosomal DNA is shared

Since X-DNA segments can be carried in larger segments through more generations, we can assume that many very small X-DNA segments will be impossible to trace because they could possibly go much further back than the autosomal segments. Most of us do not have very accurate information about our family trees going back further than ten generations,

So, it is recommended to only focus our research on X-DNA segments that are at least 15-20 cMs (centimorgans) in length, especially when no autosomal DNA is shared. In addition, we can also roughly “double” the number of generations back to the shared ancestor based on the total shared X-DNA compared to how we would estimate generations to common ancestor using autosomal DNA.

As a very, very general example, let’s take a 20 cM shared X-DNA segment. If it were autosomal DNA, we might estimate that this cousin could be related as a 4th-6th cousin

With X-DNA, our time is best spent assuming that a 20 cM shared X-DNA segment is an 8th cousin or more distant. There will certainly be instances where we can identify our relationship to a previously unknown 8th cousin, but we might find it easier to focus on larger X-DNA segments to identify closer relatives.

It is important to note that the lack of shared autosomal DNA does not necessarily mean that the X-DNA match is related very distantly, it only means that small X-DNA segments will be more difficult to trace because we do not have the additional information provided by shared autosomal DNA.

Does the X-DNA inheritance pattern affect size of shared segments?

The size of shared X-DNA segments, and whether or not any X-DNA is passed down from one generation to the next, is greatly affected by the unique X-DNA inheritance pattern. Depending on the biological gender of the parent and child, the X-DNA chromosome inherited by the child from a parent may not go through the process of recombination.

For those who are unfamiliar with how X-DNA is inherited, let’s quickly cover the basics. Males inherit an X-chromosome from their mothers and a Y-chromosome from their fathers, and they do not pass down their copy of the X-chromosome to their sons.

Females inherit X-DNA from both of their parents, which means that they have two copies of the X-chromosome. When these females pass their X-DNA to their children, both male and female, their two X-chromosomes must “recombine” to make a brand-new chromosome of the correct size to pass to their children.

Since fathers only have one copy of the X-chromosome, the copy that they pass down to their daughters is exactly the same. There is no second copy of the X-chromosome in males that needs to recombine, so X-chromosome, which was made from the father’s mother’s two copies of the X-chromosome, gets passed down “intact” to the daughter.

Of course, once the daughter has children, her two copies of the X-chromosome will typically recombine to be passed down to her children. It is possible for one of a female’s X-chromosomes to pass down without recombination, which also can make estimating how old a shared segment is more difficult.

To summarize, the way that fathers pass X-DNA to their daughters is the reason that we have to look at our X-DNA matches more carefully and assume that many of those matches are more distantly related than they might first appear.

How does X-DNA but no autosomal shared happen?

You are now familiar with the concept of recombination, which is when two copies of the same chromosome recombine to make a new copy to pass to our offspring. This happens with the two X-chromosomes that females have, but it also happens in both males and females on the other 1-22 autosomal chromosomes.

Recombination must occur because our chromosomes are not big enough to hold all of the DNA from all four of our grandparents. Portions of the DNA from each copy of a chromosome, both paternal and maternal, are randomly selected each generation to pass down to our offspring.

In addition, recombination is the way that genetic diversity is reliably obtained. It would be no fun if we were all the same as our siblings!

After a number of generations, much of the DNA from a distant ancestor is no longer detectable in our DNA. This process works the same way for autosomal DNA that is does with X-DNA, but since X-DNA has a bit more “staying power” (for lack of a better term!), due to the inheritance patterns I mentioned above, we can often find these X-DNA matches with whom we share no autosomal DNA.

Assuming a legitimate X-DNA match, if we were able to have access to ancestors from previous generations in the family tree, we would eventually find relatives that still shared autosomal DNA with each other.

In fact, I can think of a few DNA matches in my own X-DNA match list on Gedmatch that share large portions of X-DNA with me, but no autosomal DNA. When I compare them with my dad and grandmother, however, there is still shared autosomal DNA between them.

Conclusion

I hope that this post has helped answer your questions about how it’s possible to share X-DNA with a relative but no autosomal DNA. It’s a very interesting phenomenon, and now you know how it happens and how to interpret these matches.

If you have any questions about something that you read in this post, or if you would like to share an example of a match like this, please join in the discussion below.

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