Sibship Testing

Sibship DNA testing is conducted in order to determine if two or more children share one or both biological parents in common. Full siblings will have both parents in common, where as half siblings will have one parent in common, either their mother or father. When an alleged father is not available for a paternity test, sibship testing is one way to determine family relationships. Results of a sibship test have also been accepted as proof in Social Security benefit and other inheritance claims. In this type of testing, the DNA profiles from the potential siblings are compared to see how much of their DNA could have come from either one or both common parents. If the paternity of the siblings is in question, the participation of the mother(s) is strongly recommended in order to help exclude the mother's contribution to the children's DNA. Sibship tests require the analysis of more genetic markers then the number typically utilized for traditional paternity testing. Sibship testing can be more costly and will take a longer time to complete as compared with a standard paternity test.

Sibship DNA tests determine the statistical probability of whether brothers and sisters share the same two parents or not. Unlike a DNA paternity test which will always provide a conclusive result, the results of a sibship test are different, and may not provide the definitive answer the tested parties desire. When a sibship test is conducted, the DNA laboratory will determine the genetic profiles of the alleged siblings. Based on the type of genetic variations inherited by each sibling, a sibship index is calculated. A sibship index less than 1.00, provides genetic evidence which does not favor a sibling relationship. However, if the sibship index is greater than 1.00, the genetic results suggest that the two tested individuals are more likely to be true biological siblings. The higher the value of the sibship index, the greater the likelihood that the two individuals are true biological siblings. To analyze the possibility that the siblings share only one common parent versus two common parents, a half sibship index is also calculated. The ratio of the half sibship index versus the full sibship index would provide the participants with an idea of whether the two individuals are more likely to share only one common parent (e.g. mother only) or two common parents (mother and alleged father). Although the sibship DNA test may not provide you with a conclusive answer, it may be able to provide you with an indication of whether you are more likely to be or not to be the true biological sibling of the other tested individual. UNTHSC strongly recommends that individuals requiring sibship testing consult with one of our directors to explain the potential limitations of this type of DNA analysis.