Multiple mutations, covert mutations and false exclusions in paternity casework

• Published in: International Congress series 2004-04, Vol.1261, p.112-114
• Main Author: Brenner, C.H
• Format: Article
• Language: English
• Subjects: Covert mutation; Exclusion; Mutation; Paternity index; Stepwise
• ISSN: 0531-5131; 1873-6157
• DOI: 10.1016/S0531-5131(03)01843-0

The universal practice, up to now, is to make the judgement “paternity excluded” whenever there are more than some established number—such as two—of loci in which the genetic pattern, barring mutation, is inconsistent with paternity. Such a rule is founded on the implicit assumption that the probability of two mutations is vanishingly small. However, the ideal procedure would of course be to evaluate the paternity index (PI) over all loci, taking possible mutation into account. With STRs, unlike with RFLPs, a reasonably accurate mathematical model of mutation exists and hence the ideal procedure is finally possible. What happens when it is applied is somewhat surprising. Notwithstanding two or even three inconsistent loci, the posterior probability of paternity (assuming 50% prior probability) can easily be 20%. Unless the inconsistencies are particularly implausible as mutations (i.e. multiple repeat units), the posterior probability is not vanishingly small. The old rule causes bad decisions; it excludes fathers. Instead, we should compute the proper paternity index across all loci, considering the possibility of multiple mutations, and evaluate the result. The computing part is easy. The evaluation part brings a new difficulty, for it forces us to confront a question that the inaccurate policy of the past hid from view: How unlikely must paternity be in order to justify the decision “paternity excluded”? An incidental discovery is the heretofore overlooked implication that the existence of “covert mutations” imply that most STR mutation estimates from paternity studies are wrong.