Analysis of recombination and mutation events for 12 X-CHR STR loci: a collaborative family study of the Italian Speaking Working Group Ge.F.I.

According to ISFG guidelines on the use of X-chromosome, the biostatistical evaluation in kinship analysis is based on a likelihood ratio approach, but linkage and recombination events should be accounted for in calculation, therefore it is important to have an accurate estimate of haplotype frequencies of the reference population, as well as the knowledge of mutation and recombination rates of X-markers analyzed. The increased demand to forensic laboratories for kinship investigations in complex cases explains the need to increase the Italian population database for 12 X-STRs routinely used for forensic applications. Indeed, due to the mode of genetic transmission and physical location, sometimes X-chromosome markers can be more informative than autosomal STRs and their analysis could be considered a supplementary tool in DNA testing. A collaborative exercise involving 17 laboratories of the Italian Speaking Working Group Ge.F.I. was organized to evaluate mutation and recombination events in 12 X-STRs included in the Investigator Argus X12. In order to explore the segregation stability, 50 three-generation families (grandpa-mother-son) and 109 two-generation families (mother-sons) were analyzed and calculation to estimate the recombination fractions between pairs of markers and mutation rates were performed. Evidence of mutation as well as inter- and intra-cluster recombination events were observed. Moreover, 1042 unrelated male individuals from different regions of Italy were typed to expand the existing Italian X-chromosome haplotype frequencies database. Population genetic parameters of forensic interest and genetic distances between the Italian sample and worldwide populations are reported.