I have found somewhat contradictory information on various typically good sites...
Special Treatment for X Matches from
While the generally accepted threshold for autosomal DNA is about 7cM, for X DNA, there appears to be a much higher incidence of false matches at higher levels than the rest of the chromosomes, as documented by Philip Gammon as in his Match-Maker-Breaker tool. This appears to have to do with SNP density.
I would encourage genetic genealogists to consider someplace between 10 and 15 cM as an acceptable threshold for an X chromosome match. This of course does not mean that smaller segment matching can’t be relevant, it’s just that X matches are less likely to be relevant at levels below 10-15 cM than the rest of the chromosomes.
But the entire article is written as looking at X as an enhancement to typical matching on the 22 chromosomes. So it is inferred but not explicitly stated that you have to first match at 7cm or above to a chromosome.
robertajesteson May 9, 2016 at 3:42 pm said:
Family Tree DNA only shows X matches if there are other matches as well over the match threshold. X matching alone, especially without other matches, can be deceptively high and from very far back in time, which does not really play by the same rules as the rest of the autosomes. I’m assuming that is why.
robertajesteson June 4, 2015 at 8:57 pm said:
I would rely on other information from other chromosomes primarily. I have found the transmission characteristics of the X to be somewhat different than the rest of the chromosomes so I could be hesitant to apply the same guidelines to the X as to the other autosomes. I don’t believe Blaine Bettinger included the X in his cM study, but it would be very interesting if he were to do so.
robertajesteson June 3, 2017 at 10:01 pm said:
Keep in mind that X chromosome fragments generally speaking need to be about twice as large (cM) as other chromosomes to be a valid match.
- X-DNA is different. The cM match amounts are not very helpful in determining how closely related you are.
- The X Chromosome is not passed from father to son, so the lines between you and a strong (over perhaps 4cM) X-DNA match and your common ancestor will not have any father-son relationships. You'll notice in the screenshot above that I (being male), as expected, have 0cM X-DNA match with my paternal grandmother and 69.7cM match with my maternal grandmother.
- If you are a male, any X-DNA matches will be related to you on your mother's line. If your X-DNA match is male, you will be related to him via his mother.
You can read more about X Chromosome matching at http://smithplanet.com/stuff/x-chromosome.htm.