I have an X question

[Gary Corbett]

Some quick background-
I have a 4th cousin in North Carolina that I know very well.
We are related through our fathers,going back to a Massengill/Corbett marriage in 1815.
My 23andMe accounts include myself,both parents,my cousin and her paternal aunt,who would be my dad's 3rd cousin.
Anyway,my mom also shares a segment with the aunt,though we don't know why.Not a big surprise,as my mom has a couple of lines that went through Johnston County.Nothing like my dad's though,as that is where he was from.
So just today I got a match with a man named Garis.
Raised eyebrows,as that is a name my cousin has and researches,though spelled Garris.
The interesting thing to me is this match is through my mom not my dad.
Guessing possibly the Garis family could be the connection between my mom and some of my dad's people.
It shows my mom having 17m on the X chromosome half identical with this new match.
It also shows me having 17m half identical with him,but also 18m completely identical.
I don't know enough about how they are reporting this to snap to how this could be.
Anyone care to enlighten me?
Also,none of my other accounts shows any matching with this guy,though we all know that does not exclude anyone from being related,for several reasons.
Thanks for any help.

[kjjohnston]

Hi Gary,
At 23andMe you will be completely identical on the X chromosome if you are both males. If you are using the genome browser in the Family and Friends, Family Traits section, any male who matches another male will be considered fully identical even though you each only have one chromosome. Two males who match on the X will always show a black color at that site. A male to female match will show half-identical and be colored blue. Your match on the X can only come through your mother as you have stated. 17 cM could be quite far back in time. Identical-by-State sequences can add 1 cM to equal 18.
See: http://www.thegeneticgenealogist.com/2009/01/12/more-x-chromosome-charts/
The probabilities that are listed here are helpful because you and your cousin can focus on your mothers' lines that are most likely to be successful. Genome Mate can help if you have GEDcoms.
You won't show up as matches at Family Tree DNA if this is an X only match.
Kathy

[Gary Corbett]

Thank you Kathy!!
Definitely stuff I didn't know.