When is an X chromosome test used?

Sisters may be unable to carry out a paternity DNA test if the father is deceased or unwilling/unable to be tested and in such cases an X chromosome test will allow you to establish whether you are:

  • Half sisters or full sisters
  • Half sisters or not sisters at all

Establishing paternity through X chromosome testing

If you wish to establish paternity, there is no test that is more clear cut and straightforward as a paternity test which directly compares the alleged father’s DNA to that of the child. It is important to note that we always advise doing a home paternity test where possible rather than the X-chromosome test as the testing procedure for the X chromosome is more complex and thus, more expensive.  Results for a paternity test are also more direct and conclusive.

The X chromosome test is based on the fact that females receive one X chromosome from the mother and the other from the father. Therefore in the case where two sisters have different mothers, we can do an X chromosomes test without the need for the sample of the mother. The X chromosome test will therefore seek to establish whether they share the same X chromosome, thus concluding they share the same father.

X chromosome testing cases requiring the mother’s sample

Two sisters that share the same mother and want to establish whether they are full siblings or half siblings will need to include their mother’s DNA sample alongside their own to be able to conclude the test. We will need to map the X chromosome pattern in the mother’s DNA sample and exclude this from the DNA profiles of two sisters. We can then compare the remaining X chromosome pattern on the sisters’ DNA – if it matches, it will indicate that they likely inherited it from the same father. If the mother’s sample is not available then a siblings STR test is recommended.

X chromosome testing cases not requiring the mother’s samples

If the females being tested have a different biological mother, we can carry out the test via direct analysis of the DNA samples of the females involved without the need for inclusion of the mother’s samples.

Testing 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.

The cost of an X chromosome test for two sisters is $399 and your results will be ready in 15 working days. You can proceed to order your test online or if you have any questions you can contact us or request a call-back free of charge!


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