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When you test your autosomal DNA at any of the five genealogy DNA testing companies (ie. Ancestry, 23andMe, MyHeritage, Family Tree DNA, Living DNA), your X-chromosome is also examined and X-DNA is included in your raw data &/or match results.
When you match someone on your autosomal DNA and you also share segments on the X-chromosome, the unique inheritance pattern of X-chromosomes can help limit your common ancestor to particular lines of your ancestry.
Family Tree DNA‘s Family Finder test identifies X-Matches with people you already match via your autosomal DNA, and displays them in your match list (see 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).
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.
MyHeritage DNA does not yet show X-matches, but they announced at their Nov 2018 Live Conference that X-matches would be coming in future. You can upload a copy of your MyHeritage DNA raw data to FTDNA or GEDmatch for X analysis.
Living DNA does not yet report on X-matches, but will likely do so in the future when all their matching and features have been fully implemented. You can upload a copy of your Living DNA data to GEDmatch for X analysis.
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.
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):
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-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.
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.
With no X-DNA inherited from some ancestors, varying amounts of X-DNA inherited from others, the randomness of DNA recombination at each generation, and occasional segments passed down intact over several generations, X-DNA can be quite unpredictable and it can be difficult to identify 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 (or may just be too far back).
For X-matches on FTDNA’s Family Finder, use the chromosome browser to check the segment sizes, as FTDNA includes very small segments (down to 1cM) in their total cMs, and you want to exclude those tiny segments and only count segments of 7cM or more.