Sibling DNA Test Results Explained

August 21, 2017

Sibling DNA testing is performed when two individuals want to determine mutual parentage. Therefore, it falls under the category of relationship DNA tests. It is most commonly used to determine whether the said individuals share the same father.  

A special category of sibling DNA testing is reserved for the so-called “twin zygosity test” designed to determine whether twins are fraternal or identical. The results of this test are very reliable and have contradicted initial ultrasound examinations on numerous occasions.  

Sibling DNA Test Results

Types Of Sibling DNA Testing

The type and final results of your sibling DNA test will be determined by the question you asked before your DNA sample was processed. Just to illuminate the topic before we go any further, “full siblings” have the same biological parents while “half-siblings” share only one biological parent. The third potential outcome of the test is “unrelated,” which is pretty self-explanatory. On average, full siblings share 50% of their genetic material while half-siblings share 25%.

So, based on your initial question and the particular parental relationship, your test and the final results will fall under one of these types: 

  • Full siblings vs. Unrelated – The test compares the DNA of two individuals to determine whether they share biological parents or are completely unrelated. This is the most frequent type of sibling testing in US immigration cases.
  • Full siblings vs. Half-siblings – This type of test determines whether two individuals with the same mother also share a father. It is highly recommended to also include the mother’s DNA sample into the process. This will enable your chosen provider to determine which genes were inherited from the biological father and increase the overall conclusiveness of the final report.
  • Half-siblings vs. Unrelated – This test will compare DNA samples of two individuals with different mothers to determine whether they have the same father. The DNA samples from both biological mothers should be included into the testing procedure for the same reasons as with the previous testing iteration.

Based on your initial inquiry, your test results will say one of three things: full siblings or unrelated, half-siblings or unrelated, and full siblings or half-siblings.  

Is Sibling DNA Testing Conclusive?

Is Sibling DNA Testing Conclusive

It’s important to realize that sibling DNA testing yields less conclusive results than maternity or paternity testing. This is quite logical when we apply some solid reasoning to the situation. Namely, the degrees of shared genetic markers and patterns of inheritance are different between siblings than between parents and their children. The degree of certainty in sibling DNA testing can be heightened by introducing more undisputed siblings and close relatives into the processing pool.  


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