Mutations on the X chromosome are rare but when these do happen they can have an effect on your X chromosome test. The X chromosome test is often used to know if two or more females share the same biological father. The test is highly accurate and when no mutations are present, gives a clear-cut “yes you share the father” or “no you do not share the same father”.
When the father is unavailable for a DNA paternity test, females wishing to determine whether they share the same dad (which could make them half siblings, full siblings or not siblings at all) can opt for a relationship DNA test; ideally the X test. In this test the X chromosome profile is mapped and all the genetic markers displayed on the DNA test result. If the profiles between females are a perfect match, then undoubtedly they share the samedad. If the test involves only males, then the Y chromosome test would be the recommended test to do.
We do have to consider the possibility of mutations arising on the X-chromosome that would result in the different regions appearing as mismatches for a case of genuine paternity/relatedness. In this case, the presence of two mismatches is not sufficient to exclude the two individuals from sharing a father; however, the odds that two mutations would arise over one generation (especially for males donating an X-chromosome) are very low. In this case an inconclusive result will be issued. An inconclusive means that no conclusions can be drawn and whether the two females share the same father remains unanswered.
How frequent are mutations on the X Chromosome?
The frequency of inconclusive X-tests is extremely low. Out of every 100 X chromosome test carried out, less than one of these tests will give an inconclusive result.
If the X chromosome test result shows more than three mismatches than this would be a clear exclusion- the females do not share the same dad.