[Frequency quotient of paternity index Qf in a case of replacement of the natural father within a trio with his brother].

Cases of trios in which the natural father's near blood-relative instead of the natural father are among the hardest to give an expert opinion on. The goal of this study was to determine with what frequencies specific given values of paternity index appear in trios with the natural father and with the natural father's brother, and whether these frequencies can be the basis for discovering the substitution of the natural father by a brother in a given trio examined. The study material was the population of genotypes investigated in the AmpF/STR SGM Plus system. It was seen that the higher the paternity index values, the more frequently they appear in trios with the natural father, and less frequently in trios with the natural father's brother. This dependency has a general character and obtained in all the systems studied. The frequency quotients Qf (defined as the frequency of a given PI value in trios with the natural father in relation to the "reduced" frequency of the PI value in trios with the brother of the natural father) are found in the region of c. 0.6 to c. 1.4, whereby extreme values are rare, and the most frequent are cases where Qf oscillates within modest limits around the value 1. This fact shows that the majority of cases, calculated Qf values contribute little to resolving the question of whether the defendant--or his brother--is the child's father.