Tip 11: Autosomal Matches with large X matches.

Sample: Autosomal match segment 27.5 cM, X match 19.4 cM from GEDMATCH
The reason that a match of this size [25+ cM] is nice, but not exciting is that they have too many ancestors to work with. If this is a 5th cousin, you have a possible shared ancestor who is one or two of 128 ancestors. And it could be even worse. Remember that your goal is to find a common ancestor. This is a really depressing number. But if you could cut down the ancestors by half, it would really help out.
If you have a tiny X match you run the danger of it not being an actual match. Those of us with Colonial Roots etc, know that little 3 cM X segments are difficult and may not even be IBD. But a larger x segment of perhaps 7 or 8 or 9 cM is extremely important. On the whole, I am suspicious of anything much under 9. Note that this is smaller than I require for a true autosomal match.
All of the first 22 chromosomes have wo sides, and one comes from the mother, and one from the father. On the 23rd Chromosome, the so-called sex chromosome, a male receives a non-combining Y Strand from his father and a recombined/or exact X strand from his mother. So, any X he has comes from his mother. This is absolute. Bear in mind that it is possible to have related matches on both sides. But he got the X from his mother. So, if you have a decent X match to a male, the match is through his mother.
Females on the other hand, receive the exact X strand from their father that he got from his mother. They also receive an X strand from their mother which might be a recombined or an intact strand.
This off-center inheritance allows us to follow a pattern. We have charts of female and male X.
Use the male one if your match is male. Or female if your match is female. You don’t have to use the charts but when you are just beginning this, you may find your head starting to spin if you don’t.
If you have a fourth cousin match with an X match, you have narrowed the possible common ancestors to 8 from 64. If your match is female, you have narrowed the number of possible ancestors down to 13 from 64. If you have a male 5th cousin with a significant x match, you have narrowed the possible ancestor list down from 128 ancestors to 13. In other words, a fifth cousin suddenly becomes useful.
If you want to read about the X Chromosome on a blog, I think Blaine Bettinger’s article is good. He also has the same charts that appear everywhere else.
http://www.thegeneticgenealogist.com/2008/12/21/unlocking-the-genealogical-secrets-of-the-x-chromosome/
Read what he says, but I like these charts the best.
http://www.genealogyjunkie.net/x-dna-inheritance-charts.html
Bear in mind that smaller X matches may come from another match which is back in the 1600s where you will never find it. X changes more slowly than the rest of your autosomal DNA, but on the other hand, it might come from someone closer. It is at least a good idea to check it out.
Next you need a tree for your match. I prefer to work from paper. You can do it on the computer if you choose. Go through the tree with the chart in your hand, drawing a red line through any impossible ancestor. For example, a male could not have inherited that 19.4 cM match from his father, so we mark out the father and all of his ancestors. Now the mother could have gotten it from either parent. But her father could not have gotten it from his father, so you mark him out with all of his ancestors.
Eventually, you are left with only part of his tree to investigate more fully. You may also find someone else who matches both of you on the X and if you, do this is truly useful. I would take any other matches with X matches to you and run them against this match. Use the Gedmatch.com X one-to-one tool.
I found my father with one of those charts. A 2C1R match arrived on 23andme with a large x on 23andme. He had an unusual name and was born in 1939. He would not talk to me, but I went and found him in the census and built his tree out and started working the X chart out and I had my father in a couple of hours. Of course, that was because his tree lurched into another tree I had been working on in great confusion for two months. The even better news about doing this is that you can track the X down from the Common Ancestor and narrow down where you belong. You just have to narrow down the possible people. No, I did not say this was no work. I just said it is doable.
So an X of that size is very useful. I do not exactly ignore the smaller cM X matches, but I personally have not found them to be of much use from a practical point of view. I can never be absolutely certain that they are IBD and not IBS.
Because of the odd inheritance, sometimes you have to see it to tell how you are related. Two half-sisters who share an exact strand of X are probably paternal sisters. Usually, they would get a recombined X strand from their mother. Two half-brothers who have no X probably are paternal half-brothers. Long, long segments of X DNA on your mother’s side are usually from her father. And so on it goes. Feel free to post any matches you are looking at and maybe we can help. Because the male DNA is not combined when it is passed down, it does not break up as fast.
Also please be careful in endogamous communities. I have seen men who have X matches to paternal matches in the AJ community. Well, yeah. Everyone is related.
We do not usually start out with searching for an X match, but if that is what you have, that is what you have. Not everyone has a beautiful 1st cousin who hands over their whole tree and DNA and talks to you. Most of us have to make do with what we have. And an X match is better than no X match.
June Byrne
DNAAdoption.org