How the X chromosome test works
The X chromosome test is based on the fact that a female will inherit one X chromosome from the mother and another one from the father. Having different mothers therefore means that they will be half-sisters only if they have an X chromosome in common (which would be the one the individual’s tested inherited from the father). In case in which the X chromosome does not match, this would indicate that they do not share the same biological father.
In cases where the sisters share the same mother, we will need to carry out an X chromosome test with the mother’s sample so as to have a clear idea which common X-chromosome was inherited from the mother. Once we have established this, we will then be able to determine whether they share the same father or not. If the mother in common is not available for testing, then we recommend that you do a full siblings DNA test.
Paternal grandmother and granddaughter
The X chromosome test can be used between paternal grandmother and granddaughter as they must share one X chromosome. This shared X chromosome is the one that the grandmother passed to her son and he, in turn, passed to his daughter. We only need DNA samples of the paternal grandmother and granddaughter for this test. We require no other samples in this particular case.
If two females wish to establish if they share the same biological father, they can opt for a home paternity test, which offers far more accurate results. Results of a home paternity test will give you a Yes or No answer and other relationship DNA tests should only be considered as alternative but secondary option in instances where it is not possible to get a DNA sample from the alleged father.