When the father of a child is not available for testing the grandparents can be tested.  This test is possible because if the grandparents are the biological parents of the biological father of the child then they possess the paternal genetic markers of the child in question.  The genetic analysis for a grandparent test is the same as when conducting a standard paternity analysis.  For cases when the grandparents are not excluded, a probability of grand parentage is determined.

When the mother of a child and both grandparents are tested along with the child then the power of this test is comparable to a paternity test that includes the alleged father.  If the mother is not available for analysis the test is slightly weakened and is comparable to a standard paternity test when the alleged father is tested but not the mother.  If only one of the grandparents is available for testing then the power of the test is substantially weakened.  When only one grandparent is tested it becomes impossible to exclude the child as the grandchild.  This is because if the tested grandparent does not contain the required paternal genetic marker, the paternal marker may be present in the absent grandparent.