The X chromosome test is frequently done to determine the existence of biological relationships between females. The test is one the many relationship DNA tests; these tests are very specific and also use a different DNA testing methodology when compared to the DNA paternity test.
Females have an XX chromosome pair and males have an XY chromosome pair. A female inherits one X chromosome from her father and one from her mother. A male inherits one X chromosome (only possible from his mother of course) and the Y chromosome from the father.
The X chromosome test is done between females who wish to determine whether they share the same father. Including a known mother’s sample helps especially if they have the same mother. The X chromosome test is done in the place of a paternity test when it is not possible to get any DNA samples from the father. Typically, the father is dead or just unavailable for the test.
Testing of the X chromosome will trace the X chromosome patterns between the females involved to determine whether they are half sister, full sisters or not sisters at all. The costs of the test will of course, as in other DNA test, depend on the number of females being tested.
Siblings, either male or female, wishing to establish whether they share the same mother (perhaps adopted children) would do a DNA maternity test or even more accurate an MtDNA test (mitochondrial DNA test).
Relationship testing, as in the example of 2 females doing an X chromosome test to see if they share the same dad, will never be as accurate as a paternity test. The X chromosome test will show the probability of relationship between two females.