What does it mean if I share an X DNA match, but no autosomal genes? Read this post to understand if these individuals could be true genetic relatives and how you can detect your relationship.
Finding an unlikely DNA match that shares an X-DNA segment but none of their autosomal DNA segments is often unexpected and disconcerting, made all the more so when considering how X-DNA inheritance works.
Are these real matches? If that is the case, how far back must we look in order to locate their common ancestor?
These are excellent questions, but you might first find it challenging to locate these types of DNA matches. Ancestry does not show us X-DNA only matches, while 23andMe only displays our 1500 or so closest genetic matches.
How to Find X-DNA Matches Without Autosomal DNA DNA testing companies don’t display matches that share no autosomal DNA due to minimum thresholds for reporting DNA matches; so many people turn to other means in order to find genetic relatives through only their X-DNA.
Gedmatch offers the easiest and fastest way to search for potential X-DNA matches.
Gedmatch’s One-to-Many Comparison Tool results can be used to display your top 3000 X-DNA matches; many will not share autosomal DNA with you.
As I used this tool for this article, I discovered that there are at least 3,000 matches with only my X-DNA sharing on Gedmatch. Are these individuals really related?
Does sharing X-DNA without autosomal DNA count as real matches?
It is possible to have many relatives with whom we share only X-DNA; we typically become aware of them through DNA testing.
DNA matches who share only X-DNA could still be legitimate matches and be related through some shared ancestor; however, X-DNA segments should also be subject to the same scrutiny applied to autosomal DNA segments.
Smaller shared X-DNA segments may be false positives while larger shared segments indicate a more recent common ancestor; however, when considering shared X-DNA segments we should also take additional information into account.
Due to X-DNA’s special inheritance pattern, segments do not break apart quickly across generations like autosomal DNA does. Therefore, many experts suggest we should set a larger threshold size when using X-DNA segments than we would for autosomal DNA in order to minimize wasted time identifying segments with roots too distant for accurate identification.
Additionally, due to the nature of X-DNA testing, there is a higher likelihood that small segments that appear identical may only be identical due to chance. Such false matches cannot be traced back to any shared ancestor; rather they only appear identical by chance.
As is well-documented, most autosomal DNA segments smaller than 6cMs are false segments (i.e. identical between states). When these very small DNA segments do exist and can be passed along legally through inheritance from 8-10 generations back on our family trees or even further back.
How far back should X-DNA matches go back when no autosomal DNA matches exist?
Given that X-DNA segments can be passed along over multiple generations, many very small X-DNA segments will likely remain difficult to trace due to going further back than autosomal DNA segments. Unfortunately, most people lack accurate family trees which go back more than 10 generations; we should therefore assume it would be impossible to trace all X-DNA traces going further back.
Research should focus only on X-DNA segments that are at least 15-20 cMs long when no autosomal DNA is shared. Furthermore, using shared X-DNA we can roughly “double” our estimate of generations back to a shared ancestor when compared with using autosomal DNA alone as our measure.
As an extremely general example, consider two individuals with 20 cM shared X-DNA segments; if this were autosomal DNA we might estimate they are 4th-6th cousins.
With X-DNA, our time is best spent assuming that a 20 cM shared segment represents an 8th cousin or more distant relative. While there may be instances when we can identify newly unknown 8th cousins directly, larger X-DNA segments might provide greater clarity into more intimate relations.
Note that lack of shared autosomal DNA does not equate to distant relations; rather, it simply means that smaller X-DNA segments will be harder to track without this additional information provided by shared autosomal DNA.
Does the X-DNA inheritance pattern influence shared segments and whether any are passed from generation to generation? Yes. The pattern of inheritance for an individual child and parent can have a great influence on whether any X-DNA chromosome from either may not undergo the process of recombination as expected by scientists.
For those unfamiliar with how X-DNA is passed down, let us quickly go over the basics: males inherit an X-chromosome from their mothers and Y-chromosome from their fathers and do not pass along their copy of the X-chromosome to their sons.
Females inherit two copies of X-DNA from both parents, giving them two X-chromosomes each. When females pass down their DNA to both male and female children through reproduction, both sets must recombine into an even larger chromosome that can pass onto its progeny.
As men only carry one copy of the X-chromosome, when passed down to their daughters it remains unchanged. With no second X-chromosome for recombination purposes in males, the original two copies from their mother are passed intact to their daughter.
Once the daughter gives birth, both copies of the X-chromosome typically recombine and pass to her offspring; however, one could pass without being recombined at all, which would make estimating how old a shared segment is more complicated.
As mentioned previously, fathers pass down X-DNA to their daughters is one reason we must pay closer attention when looking at our X-DNA matches and assume many may be more distantly related than initially suspected.
How is X-DNA passed on but no autosomal DNA shared?
Most people are familiar with recombination, the process by which two copies of one chromosome combine into one new copy that we pass down through generations. Recombination occurs naturally with females having two X-chromosomes but also occurs on all 1-22 autosomal chromosomes that make up our genomes.
Recombination occurs because our chromosomes cannot contain all of our grandparents’ DNA at once; pieces from each copy, paternal and maternal, are randomly chosen each generation for transmission to our offspring.
Recombination is also crucial in creating genetic diversity; without it, life would be much duller! Imagine living alongside siblings that all looked the same!
Over several generations, most of the DNA from distant ancestors no longer detectable in our DNA is no longer detectable in us. This process occurs for both autosomal DNA and X-DNA; however, due to inheritance patterns described above we often encounter matches on X-DNA but no autosomal DNA matches!
Assuming we had access to previous generations in the family tree and could validate an X-DNA match, if we could gain access to past relatives we might eventually uncover relatives that still shared autosomal DNA with each other.
Gedmatch allows me to identify several DNA matches who share large portions of X-DNA with me but no autosomal DNA; when I compare their DNA against that of my dad and grandmother however, there remains shared autosomal DNA between us all.
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