Evaluating Y-chromosome STRs mutation rates: A collaborative study of the Ge.F.I.-ISFG Italian Group

A collaborative study was carried out by the Italian ISFG Working Group in order to improve the data on Y-STR mutations at the loci mostly used in forensic analysis, following recommendations of the ISFG DNA Commission. The knowledge on Y-STR mutation rates needs to be considered in the paternity probabilities, especially in deficiency cases of disputed paternity involving male offspring where the alleged father is not available for DNA analysis. Furthermore, the mutation rate represents a precious tool to estimate the local and temporal origin of a given Y-SNP based haplogroup. The sample consisted of 433 father/son pairs from paternity cases in 15 different laboratories from Italy. The biological relationship of all father/son pairs was previously confirmed by using autosomal microsatellites. The laboratories used AmpFlSTR YFiler kit (AB) and PowerPlex Y System (Promega); DYS19, DYS389I, DYS389II, DYS390, DYS391, DYS392, DYS393, DYS385, DYS437, DYS438, DYS439, DYS448, DYS456, DYS458, GATA C4, and GATA H4.1 data were collected. The participants were also asked to provide the age of the biological father and, if possible, male descendants beyond the first generation. 20 mutations were observed among all of the allele transfers in the sample (19 single step and 1 double step), and mutations in the same father/son pair were found in three cases. Locus-specific mutation rates were calculated. Forensic implication of the average age of the father as well as the number of locus deletions and amplifications were discussed.