Are rapidly mutating Y-short tandem repeats useful to resolve a lineage? Expanding mutability data on distant male relationships

Y-chromosomal short tandem repeats (Y-STRs) haplotyping has been established as the reference method for identifying males in forensic casework, and a large number of markers have been validated over the past 10 years with that process still ongoing. The haploid nature and the absence of recombination of the Y chromosome usually lead to identical Y-STR haplotypes among males within a defined lineage, except for rare mutations. To improve overall haplotype discrimination power, the current set of 17 short tandem repeats was recently extended to 23 Y-STRs, with the addition of four new Y-STR loci (DYS481, DYS533, DYS549, and DYS643) and two rapidly mutating Y-STRs (RM Y-STRs; DYS570, DYS576).4 However, even this enlarged panel of Y-STRs seems to be characterized by a mutation rate not sufficiently high to provide a more fine differentiation of close male relatives such as fathers–son, brothers, uncles, and paternal cousins. This may be relevant in crime scene investigations involving male relatives as it affects the identification of male perpetrators within the same pedigree. It seems that a potential contribution toward resolving this issue may come from new polymorphisms, called RM Y-STRs. These markers represent a promising and suitable tool for the differentiation of two closely related males, due to their mutation rate nearly 10-fold higher (approx. 1022 per locus per generation) than any currently used Y-STR (approx. 1023). While, up to now, large numbers of father–son pairs in the relative differentiation testing have been well studied,9 data regarding more distant paternal relationships are still lacking. Therefore, more precise estimates of their effective differentiation rate are needed. The aim of this study was to contribute to this purpose by typing 13 RM Y-STRs in 157 male relative pairs separated by two to seven generations and originating from Italy.