Since familytreeDNA added X chromosome matching to their family finder, all my favorite bloggers have been writing about how to use it. There is a list of their terrific articles at the end of this post.
Of course those of us who have many family members tested at 23andme have been working with the X chromosome for a while and have some thoughts on how useful it might or might not be.
- Smaller matches on the X will usually be too far back to find the relationship because the X chromosome does not get recombined when a father passes his only X chromosome on to make a daughter. Thus segments can stay together longer in time.
- A match on the X can eliminate many ancestors from an autosomal match BUT a small match on the X can be from a different line so this is not guaranteed.
- There seems to be less recombination on the X. I have heard of cases where a child got an unrecombined X from their mother. In my own family, the number of recombined segments is quite small. So it seems to me that on the female to female line the X behaves like a sluggish autosome and is not quite as actively recombined as the other chromosomes. [UPDATE 11-May-2018: the X has been shown to have a normal amount of recombination for any chromosome by a citizen science study done by Blaine Bettinger, click here for blog post by Leanne Cooper summarizing this.]
Less recombination examples
Because our maternal line (1/2 Ashkenazi, 1/2 Bavarian Catholic) is from two different population groups, my brother and I can learn which maternal grandparent we got our X DNA from just by looking at the ancestry composition for that chromosome at 23andme:
Notice that my brother got only two segments thus one point of recombination – he has one segment from each maternal grandparent. Whereas my X has two recombination points at each end. [UPDATE 11-May-2018: This is a normal amount of recombination in any chromosome of this size. See the update above]
A good blog post about phasing of one’s own X chomosome is at The Lineal Arboretum
X reaches way back in time
My brother and I share 13.9cM (1021 SNPs) on our X with my 100% Ashkenazi husband. We know there is no common ancestry in the last 300 years or so, probably even the last 500. He is of Galician Jewish origin via Vienna. Our jewish DNA line (one grandad) is Southern German Jewish for hundreds of years and we have a good paper trail. However Ashkenazi DNA is difficult to untangle due to many cousin marriages way back when.
Out of the 500 or so shares that my Norwegian descent Dad has in the 23andme database, he has only 6 matches with greater than 7cM on the X, not including known relatives. We have not found the relationship to any of them and two of them are even Canadian with no known Norse ancestry (update these now look to be related via an unknown Norwegian ancestor in Newfoundland long ago).
Here are some of the better explanations of X inheritance inheritance and helpful charts:
- X marks the spot by Roberta Estes at DNA-explained
- Fan charts of X inheritance by Blaine Bettinger at the Genetic Genealogist
- Debbie Parker Wayne, “X-DNA Inheritance Charts ,” Deb’s Delvings Blog, posted 25 October 2013