Full siblings impersonating parent/child prove most difficult to discredit with DNA profiling alone.

DNA profiling is currently the most widely used method for parentage verification, although many forms of it have limitations of some sort. In this paper, a general formula is derived to depict a simple relationship between the probability that a random man and the probably that a male relative of the child, other than the child's father, is excluded from paternity, when the phenotype of the child's mother is unavailable. With this, the possible limitations of a finite set of STR loci in excluding close relatives of the child from paternity are illustrated. Genetically, among the commonly encountered biologic relationships, to exclude a full sibling of the child from paternity if they pose themselves as father and child remains the most difficult.