Friday, January 3, 2014

FTDNA now shows matches on the X chromosome

Great news this week that FTDNA is now showing the X matches.

CeCe Moore has written a great post on this, so I'm not going to repeat all the details here.  You can read about that here at the Your Genetic Genealogist blog.

This is an option that has been asked for for years by genetic genealogists, and now we have it.  But I find it is causing a great deal of commotion and confusion!

The X chromosome is special, you can read some more about how and why the X is different than the other 22 chromosome pairs in my post Discussing the X.

Please make sure you fully understand (at least as best as any of us do at this point, genetic genealogy is a constantly changing game!) the "rules" of X chromosome inheritance.

It appears that FTDNA is showing all matches with even tiny X matches as X matches.  This DOES NOT mean that the match is only an X match.  For example, if you match someone for 20 cM on chromosome 1 and for 2 cM on the X chromosome.  If you are male, an X match means that the person is on your mothers side.  But, with the type of match I just described, the X match is so small that it is likely not IBD, and if you limited your search for a common ancestor to the maternal side you run a very high risk of not ever finding the correct common ancestor for the match on chromosome 1, which is likely FAR more recent than the one on the X.  

Personally, I would ignore any X match smaller than 10 cM.  Even if the X match is larger, if you have another chromosome match on another "regular" ie. not X chromosome, the two segments could be from different ancestors.  I see 2 segment matches where one is from each of my parents quite often.  I am pretty good (if I do say so myself) at finding the common ancestor with matches, but I have yet to succeed in finding an X match common ancestor, and on some of my kits I have X matches over 35 cM! 

So, while I think you should explore your FTDNA X matches, there does appear to be some glitches in the FTDNA X matching system at the moment, and I would proceed with caution is considering what an X match means in your search for a common ancestor.

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