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Who Tests the X Chromosome?

Roberta Estes

Recently, someone asked which of the major DNA testing companies test the X chromosome and which ones use the X in matching. How does this difference influence the quality of our matches?

Vendor X in Download File Uses X in Matching X Included in Total cM Count
23andMe Yes Yes Yes
Family Tree DNA Yes Yes (if have a match on another chromosome) No
Ancestry Yes *No No
MyHeritage Yes No No
GedMatch N/A Separately No

*If Ancestry did utilize the X in matching, it wouldn’t benefit customers because Ancestry does not show segment information by chromosome.  In other words, no chromosome browser.

Family Tree DNA includes any size X match IF and only if the two people already match on a different chromosome.

GedMatch, of course, isn’t a vendor who does DNA testing, so they don’t provide download files.  They are solely on the receiving end.

X CentiMorgan Counts

Due to variations in the way vendors calculate matches and total cM counts, your mileage may vary a bit.

In other words, the 23andMe cM total, if an X match is involved, may be slightly more than a match between the same two people at Family Tree DNA, where the X match cM is not included in the cM total.

Conversely, you won’t show an X match with someone at Family Tree DNA if there isn’t also another segment on a different chromosome that matches.

In general, due to the thin spread of SNPs on the X chromosome, you will need, on average, a cM match that is twice as large as on other chromosomes to be considered of equal weight.

In other words, a 10 cM match on the X chromosome would only be genealogically equivalent to approximately a 5 cM match on any other chromosome.

X matches really can’t be evaluated by the same rules as other chromosomes due both to their SNP paucity and their inheritance path, which is why most vendors don’t include those segments in the total cM count.

X Matches

While including the X chromosome cM count is problematic, X matching can be a huge benefit because of the unique inheritance path of the X chromosome.

In the article, X Marks the Spot, we discussed the inheritance path of the X chromosome for both males and females. Females inherit an X chromosome from both father and mother, which recombines just like chromosomes 1-22.  However, men only inherit an X from their mother, because they inherit a Y from their father instead of the X.  Therefore, males will only inherit an X from their mother, and females will only inherit their father’s mother’s X chromosome.

Charting Companion software works with your genealogy software of choice to produce a lovely fan chart where the contributors of my X chromosome are charted in color, above. You can read more about Charting Companion here.

The great news is that if you and a match share a significant portion of the X chromosome, meaning more than 15 cM which reduces the likelihood of an identical by chance match, the common ancestor (on that segment) has to come from an ancestor in your direct X path.

I’m always excited to see with whom I share an X.  That piece of information alone helps me focus my ancestor detective efforts on a specific portion of my tree.

Some X segments can remain intact for generations and may be very old.  So don’t be surprised if the common ancestor of the X segment and another matching segment may not be the same ancestor.

Sorting by X

I wasn’t able to find a way to sort by X chromosome matches at 23andMe, but you can sort by the X at both Family Tree DNA and GedMatch.

At GedMatch, X matching shows on the one-to-many match page.  You can sort by either Total X cM or Largest X cM by using the up and down arrows, at right, below, in the X DNA columns.

After you identify an X match, be sure to run the X one-to-one match option to verify.

My GedMatch matches cause me to wonder if 23andMe is using a different reporting threshold for the X chromosome, because one of my matches at GedMatch is a close family member with no X match at 23andMe, but a total of 32 X cM and with a longest segment of 14 X cM at GedMatch.

That same individual matches me with the largest X segment of 14 cM at Family Tree DNA as well.

Family Tree DNA X Match Phasing

At Family Tree DNA, on your Family Finder matches page, just click on the X-Match header (at right, below) to bring all of your X matches to the top of your list.

If you have linked any kits of relatives to your tree, you will see numbers of phased kits on the maternal and paternal tabs with the red and blue male and female icons. In the example above, I have 3313 matches total, with 744 being paternal, 586 being maternal.

Next, click on the maternal or paternal tab to see only the people with X matches who match you on the  your maternal and paternal lines. Matches are automatically sorted into maternal and paternal “buckets” for you. Remember to check the size of the X match before deciding about relevance.

Who is your largest X match that you don’t already know?  Maybe you can find your common ancestor today.

Have fun!!!

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