What is a Grandparent DNA Test?
A grandparent DNA test is one in which the genetic material (DNA) of a child is compared to that of one or two other individuals to determine the likelihood that the child is related to the other individual(s) as a biological grandchild. In most cases, grandparent tests are performed to determine paternity—whether or not the biological son of a tested individual is the biological father of a child—in situations where the possible father is deceased, incarcerated, unwilling or otherwise unavailable to participate in a paternity test. In a legal setting, test results are often presented by grandparents to obtain custody of their grandchild, by mothers to obtain Social Security benefits for their child, and by petitioners to provide evidence of biological relationship for immigration purposes. In rare cases, a grandparent DNA test may also be performed to determine whether individuals are maternal grandparents of a child, for cases in which maternity is in question but in which the alleged mother is unavailable.
How is Grandparent DNA Calculated?
Because a child inherits exactly 50% of its genes from its biological mother and 50% from its biological father, that child shares exactly 25% of its genes with each biological grandparent due to inheritance. In a grandparent DNA test performed to address paternity, we are asking the question, “Does the DNA of the alleged grandparent(s) contain the genes that the child inherited (or could have inherited) from its biological father?” If both biological parents of the alleged father are tested and they do not possess the paternal genes between them, then they are excluded (ruled out) as the biological grandparents and the probability of grand-parentage will be 0%.
If both biological parents of the alleged father are tested and they do possess the paternal genes between them, then they are not excluded (not ruled out) as the biological grandparents and the probability of grand-parentage will be more that 99%. For these tests, we recommend testing samples from both alleged grandparents and the mother’s sample whenever possible to achieve results of greatest accuracy.
Is Grandparent DNA Accurate?
It is important to understand that it may not be possible to obtain conclusive results in a grandparent DNA test if both alleged grandparents are not tested. This is because testing one alleged grandparent allows us to identify only half of the possible genes of the unavailable alleged parent. It is also important to understand that, in reporting the results of a DNA grandparent test, the laboratory assumes that the tested alleged grandparents are, in fact, the biological parents of the unavailable alleged parent. If there is any question regarding the paternity of the unavailable alleged parent, then the alleged biological grandfather should not be tested.
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By: Dr. John Taddi