Abstract
Rapidly Mutating Y-STRs (RM Y-STRs) were recently introduced in forensics in order to increase the differentiation of Y-chromosomal profiles even in case of close relatives. We estimate RM Y-STRs mutation rates and their power to discriminate between related individuals by using samples extracted from a wide set of paternal pedigrees and by comparing RM Y-STRs results with those obtained from the Y-filer set. In addition, we tested the ability of RM Y-STRs to discriminate between unrelated individuals carrying the same Y-filer haplotype, using the haplogroup R-M269 (reportedly characterised by a strong resemblance in Y-STR profiles) as a case study. Our results, despite confirming the high mutability of RM Y-STRs, show significantly lower mutation rates than reference germline ones. Consequently, their power to discriminate between related individuals, despite being higher than the one of Y-filer, does not seem to improve significantly the performance of the latter. On the contrary, when considering R-M269 unrelated individuals, RM Y-STRs reveal significant discriminatory power and retain some phylogenetic signal, allowing the correct classification of individuals for some R-M269-derived sub-lineages. These results have important implications not only for forensics, but also for molecular anthropology, suggesting that RM Y-STRs are useful tools for exploring subtle genetic variability within Y-chromosomal haplogroups.
Highlights
Small effective population size, male-specific inheritance and remarkable geographical differentiation make the human Y-chromosome one of the most used systems for exploring questions related to forensics and molecular anthropology [1,2]
In this study we explore the potential of 13 RM Y-STR markers for forensic and anthropological applications by newly estimating their mutation rates and testing their power to differentiate between related and unrelated individuals
As for RM Y-STRs, our pedigree-based estimations confirm their higher mutability with respect to Yfiler, yielding a rate equal to 1.46 Ã 10−2 [CI: 1.20 Ã 10−2, 1.73 Ã 10−2] for the full panel (RM13) and 9.57 Ã 10−3 [CI: 6.20 Ã 10−3, 1.27 Ã 10−2] after removing the most mutable loci (RM11)
Summary
Male-specific inheritance and remarkable geographical differentiation make the human Y-chromosome one of the most used systems for exploring questions related to forensics and molecular anthropology [1,2]. Y-STRs are a valuable tool to differentiate male individuals belonging to different paternal lineages. A number of criticisms have been raised, in particular concerning the susceptibility of STRs to homoplasy events [3,4]. This fact arises from the stepwise mutation model of STRs, according to which, when two consecutive mutations happen at the same locus, the latter will elide the former in half of the cases (backmutation). Identical (or very similar) Y-STR haplotypes may not be the result of a recent shared paternal ancestor, a well-known condition in forensic studies [5]. It has been hypothesized that such a behaviour may result from the combination of the recent, widespread radiation of R-M269 (which is the most frequent haplogroup in Western Europe) and the inherent tendency to homoplasy of STRs [4]
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