If the father is deceased or simply unavailable for testing, a grand parentage test is often one of the better relationship tests to perform to determine paternity of a child. The various combinations of individuals to test are as follows:
The best testing scenario is option “a” which includes the mother, child, and both of the alleged father’s parents (the child’s paternal grandmother and grandfather). By including both of the alleged father’s parents, the alleged father’s DNA can be reconstructed. Including the mother’s DNA allows 50% of the child’s alleles to be accounted for, with the remaining 50% now known to be the father’s. Comparing the alleged father’s reconstructed profile against 50% of the child’s DNA can allow this testing scenario to give results as accurate as a paternity test.
Each time a person is removed from the test, such as a grandparent or mother, the strength of the genetic evidence decreases, therefore testing all 4 individuals is the best testing option.