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When the issue of sibship is in question, the recommended method to determine whether individuals are true biological siblings is to test their parents. DNA paternity testing will provide conclusive results. However, in situations where the alleged parents are unavailable for testing, a DNA sibship test can be performed to analyze the relationship of biological siblings.
Unlike a DNA paternity test which will always provide a conclusive result, sibship tests are different. When a sibship test is conducted, the DNA laboratory will determine the genetic profile of the alleged siblings. Based on the type of genetic material inherited by each sibling, a sibship index is determined. If the sibship index is less than 1.00, this indicates non-relatedness. If the sibship index is greater than 1.00, this indicates that the two tested individuals are more likely to be true biological siblings. The higher the value of the sibship index, the greater is 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 (mother and father), a half sibship index is obtained. The ratio of the half sibship index versus the full sibship index would provide you with an idea of whether the two individuals are more likely to share only one biological parent (e.g. mother only) or two biological parents (mother and alleged father).
Although the sibship test will 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.
When should you consider a sibship test?
Determination of whether two individuals are half siblings (only one parent in common.) Determination of whether two individuals are full siblings (same mother and same father.) Method of choice in situations where the parents are unavailable for testing.