When the alleged father is a close relative of the real father: The utility of insertion/deletion polymorphisms

The occurrence of Mendelian incompatibilities between true father and child is not an uncommon situation. Thus, in paternity investigations, when only a few are observed, it is possible that the alleged father is either the real father or his close relative. In this study, we intended to test how a set of autosomal indels can complement the results obtained with a routine battery of STRs in paternity investigations where the alleged father is, in fact, a close relative of the real one. We analyzed 100uncle/nephew and grandfather/grandchild unrelated duos, through the amplification of 15 STR markers and a set of 38 non-coding bi-allelic indels. For each duo, we determined the number of observed incompatibilities and calculated the LR assuming a father/son relationship, for both STRs and indels. As expected, in a significant proportion of cases, no incompatibilities were found. In these instances, indel's LR contribution was always in favour of the “wrong” hypothesis, strengthening a false paternity. Therefore, in dubious cases, an extended battery of indels can be useful to exclude false fathers but should be taken with caution when no incompatibilities are detected. We estimated that to obtain a satisfactory probability (less than 1 in 500) of finding zero incompatibilities between 2nd degree relatives, approximately 100 indels with maximum diversity would be necessary.