Effect on paternity index of substituting the biological father by his brother—preliminary report

Abstract

Background: In cases of analysis of trios with the biological father, inconsistencies never appear (barring the possibility of a mutation occurring). The paternity index (PI) calculated for such trios always has a value greater than zero. If in such a trio the biological father is substituted by his brother, it can cause a change in PI if the brother's genotype is different from the genotype of the biological father. The purpose of this study was to determine what the effect on PI for a trio would be if the biological father in the trio was replaced by his brother. In the study, a computer simulation was used of all theoretically possible trios with the biological father and his brother. For all the simulated trios, PIs were calculated and compared. The calculations were based on SGM Plus systems. It was determined that PI calculated for the trio with the biological father's brother can only have one of four possible relations to PI for the trio with the biological father. Most often, there was no change in the PI—this occurred in over half the cases (53.7%, SD=0.9%). Significantly less frequent was the situation in which the result of the substitution was an inconsistency (32.3%, SD=3.8%). The remaining two possibilities—twice greater or lesser PI resulting from substituting the father in the trio—were very rare and appeared with medium frequency of only a few percent (4.7%, SD=1.0% and 5.3%, SD=2.0%, respectively). Substituting, for the biological father in a trio, his brother, in so far as no inconsistency appears as a result, does not cause any change in PI in the majority of cases. Thus, distinguishing the biological father from his brother can occur practically only by means of excluding the brother.