X Match interpretation

[pughcd]

Hi Ruskie,

I (male) and my half-sister have both transferred our autosomal DNA raw data to FamilyTree DNA.

I have an X-match with my half-sister. My half-sister has an X-match with me. We share the same mother. I do not have any other X-matches on the first three pages of matches. Thus I match just 1 person on the X-chromosome but have 89 negative matches.

On the other hand, my half-sister has 66 X-matches in her first three pages of  cousin matches, including me. She has just 24 negative X-matches. Her top matches, excluding me, are a paternal line that I do not share.

There is one female individual on the first page of matches who my half-sister has an X-match with but I do NOT  have an X-match with. My half-sister shares 59cM, longest block 16cM and I share 55cM, longest block 16cM with this female individual. Her family tree indicates that she has geographical origins similar to our mother. This sounds similar to your situation.

I would recommend you ask the DNA provider for their explanation. FamilyTree DNA are very proactive, if that's who you tested with.

Excellent question!
pughcd

[Ruskie]

Thanks pughcd. Yes, your X match situation does sound similar to mine, though I don't know if there would be a difference with a half sister as opposed to a father? 

Apparently females tend to get more X matches than males which may explain your lack of X matches?

This looks like it should be useful and explains things well but I find it quite complex:
http://www.genie1.com.au/blog/63-x-dna

This says that it is difficult to work out where the X match comes from. I don't know if my situation is simply explained by the fact that females get more X matches than males, which is why my father "missed out"? 

[Spike H]

Ruskie, any DNA you have including any on the X chromosome has to have come from one of your parents. If your father does not have an X Chr match with LB he can't have passed that segment on to you.
Having stated the obvious, there is a problem with X matches in that they aren't terribly reliable. Anything less than 20cM's is likely to be false.
There has been a study in which several trios of daughter/mother/father matches were examined and it was found in all cases that an X match under 20cM's with the daughter did not have a corresponding match with either parent.

[Ruskie]

Hi Spike. Thanks for that.

I am 100% sure that my connection with LB has come from my father, but I understand that X matches are not reliable. He could not have passed it to me without it showing in his own matches?

LB and myself share 15.22cM on the X match, so under the 20cM in the test you mention.

[pughcd]

Thanks Ruskie,

The article by Louise Coakley, 2015 is excellent. I have just downloaded the pdf.

pughcd