Small segments on the X; male vs. female

Kitty Cooper put out a blog post where she entitled it What Can the X Chromosome Tell Us About the Importance of Small Segments? by Kathy Johnson.   Kathy Johnson had gone through the males in her project and began analyzing and compared to females, determining how much of the females were producing false positives vs. the men.  Because not many men would get a lot of X-matches.  This seems to be an ongoing investigation with various people blogging about the validity of phasing, or rather how effective if not necessarily is it to weed out any false positive matches. It seems to be based on FamilyTreeDNA’s X-matches where they include many tiny segments as little as 1cM.  And the more substantial matches with 10cM or more tends to reduce the actual X-matches significantly, which would be due to the lack of phasing.  You can read more about it on Kitty’s blog, although most of the discussion about evaluating all of these matches took place outside of the blog and on Facebook’s “International Society of Genetic Genealogy” page.

That made me curious, because others have expressed how some men had little to no X-matches.  This was not my situation at all , and went through my list of 9 pages on FTDNA and counted 47 X-matches out of the total 89 matches that I have.  I noticed that one of them was actually an X match on my father’s side of matches, a Filipino.  I knew that was wrong.  So when I looked at it, no X match showed up in the chromosome browser until I reduced the threshold down to 1+cM where I saw a 1.9cM, a false match.

Aside from one woman mislabeled as a male in my matches, I actually have 20 men and 26 females as X-matches, not counting that Filipino false match.  That’s half of my matches.  My mother has 93 X-matches out of her 159 matches, so not that much more than me.  Could that indicate that my mother’s X-matches are more, or less of false matches?  It’s an interesting idea to see how men can have less false matches but we are looking at Polynesian matches which just adds something else to it.

I know that I do have a lot of my matches below 5cM on the X chromosome, so I used’s ADSA (autosomal DNA segment analyzer) to at least look at my ICW (in common with) matches on the X, but I had increased the threshold to 700SNPs and 10cM.

Screen Shot 2014-12-20 at 12.30.11 PM

I was thinking that not only would it be easier to use this tool by instantly seeing my X-matches above a specific threshold, but it would also compare me with others with whom we share the same segments, therefore decreasing the chances of false matches.  But taking into consideration that we are referring to Polynesians.  How would that affect it really?

I cannot determine from comparing my own to my mother’s X-matches if they would be false matches or not. Our problem, lack of documentation, lack of genetic diversity and the unpredictability of the X chromosome itself just to mention a few.

I have recently begun testing my first cousins on my non-Hawaiian side in order to take a closer look at the X chromosome and how that is passed on knowing the X path, that is how it is passed on unrecombined from father to daughter versus mother to children.  I also felt that knowing how it is passed on, it would be easier to distinguish which part of the chromosome was inherited from my grandfather versus my grandmother.  And not until I begin testing relatives from each of my grandparents’ side, I will not be able to fully distinguish all of them with the rest of the other 22 pairs of chromosomes.

Having said that, I cannot see how these X-matches, at least among Polynesians would be consist of a lot of false segments or not.  Especially when there are long segments with the more distant people, e.g., Maoris or Samoans and Tongans, of which I do have X matches with.  But the Samoans and Tongans are not included in the ICW due to the fact that I increased the threshold to exclude anything below 10cM.

I also used Gedmatch’s ONE TO MANY to get all my matches, sorted them by the largest segment on the X and just looked at how many were above 10cM.  There were only 20.

Screen Shot 2014-12-20 at 8.21.57 PM

I did the same for my brother, he got 17 above 10cM.  I also looked at other Polynesian men just to compare and the numbers varied, usually not exceeding 20 with 10cM minimum threshold.  It is still all interesting although it is hard to decipher how much of it is true for Polynesians.  Hopefully as more Polynesians get tested, we will start to notice more differences, or confirm that we just all have a high amount of X-matches.


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