A DNA sibling test is one in which the genetic material (DNA) of one person is compared to that of another person to determine the likelihood that they are related biologically as siblings. In most cases, sibling tests are performed to determine paternity—whether or not the two individuals have the same biological father—in situations where the possible father is deceased, incarcerated, unwilling or otherwise unavailable to participate in a paternity test. In a legal setting, sibling DNA test results are often presented by mothers to obtain Social Security survivor benefits for their child; in such cases, the biological father of one child is undisputed and the mother needs to prove that the other child has the same father. Sibling DNA tests are also frequently performed to provide evidence of biological relationship for U.S. immigration applications. In rare cases, such as those where a child was given up for adoption, a sibling test may also be performed to address whether another individual (perhaps located many years later), has the same biological mother.
Two individuals who share both biological parents in common are called full siblings, whereas two individuals who share only one biological parent in common are called half siblings. On average, full siblings share 50% of their genes due to common inheritance, whereas half siblings share 25%. There are 3 different types of DNA sibling tests, which are based on the particular parental relationship(s) in question:
- Full Siblings vs. Unrelated. In these sibling tests, the DNA of two individuals is compared to determine the likelihood that they have the same biological mother and father, versus being completely unrelated. This type of sibling test is most frequently requested in U.S. immigration cases, in which one individual is a U.S. citizen sponsoring an alleged sibling who is applying for an immigrant visa.
- Full Siblings vs. Half Siblings. In these sibling tests, the DNA of two individuals with the same biological mother is compared to determine the likelihood that they have the same biological father. For these tests, we recommend that samples from the biological mother also be tested. This enables us to determine exactly which genes the two children inherited from their biological father and thereby greatly increases the conclusiveness of the test.
- Half Siblings vs. Unrelated. In these sibling tests, the DNA of two individuals with different biological mothers is compared to determine the likelihood that they have the same biological father. For these tests, we recommend that samples from one—and preferably both—biological mothers be tested if possible. This enables us to determine exactly which genes the two children inherited from their biological father(s) and thereby greatly increases the conclusiveness of the test.
It is important to understand that, unlike a traditional paternity test in which samples of the alleged father and child are both tested, it is not always possible to obtain a conclusive result in a DNA sibling test. This is, in part, because the pattern of inheritance and degree of sharing of genetic markers between two siblings are not the same as that between a parent and child. In a sibling DNA test, the degree of certainty that can be achieved depends upon who is available for testing as well as the specific type of sibling relationship being tested. As a general rule, the more genetic information available, the greater the chance the test will be conclusive. In some cases, it may be advantageous to include additional undisputed siblings in the test or to test other undisputed close relatives of the alleged parent (such as a father, mother, brother or sister), as described under family reconstruction DNA tests.
For more information about our sibling DNA tests, please call us today at 1-714-648-0468.
By: Dr John Taddie