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Post date: Nov 18, 2015 5:57:38 PM
The following is a great article about the issues involved in using the X chromosome for genealogical purposes. To begin with, the inheritance of the X chromosome does not work the same as that for the autosomes (non-X or Y chromosomes - a.k.a. "the other ones"). In addition to this difference, the X chromosome tends not to recombine in a predictable pattern as the others do (the pattern of recombination by the autosomes is predictable in the aggregate). This makes the use of X chromosome matching very tricky for genealogical purposes.
In addition to the complications described in the above article, I have recently come across a very unusual phenomena. I have an individual (a male) who's autosomal and X-DNA match is neither an autosomal match nor an X-DNA match to his mother. The match is, however, both an autosomal and X-DNA match to his father.
If you have read the above article, I think that you might understand why this should not be able to happen. As a male (XY), he should have acquired his only X-chromosome from his mother, and his father could, therefore, only have donated his Y-chromosome. Therefore, any X-DNA match should be on his mother's side, and should not be able to be on his father's side.
This usual match is being looked into. My suspicion is that this unexpected match has been caused by an issue with the DNA matching algorithm that is being used by the DNA testing company. I further suspect that they will blame the test itself, or the test participant, rather than their system.
I look forward to updating this post when more information regarding this issue is available.
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