Evaluation of mutation rates and differentiation capacity of rapidly mutating Y-STRs among patrilineal relatives in a British Isles cohort

BACKGROUND: Conventional Y-chromosomal short tandem repeats (Y-STRs) have limited ability to distinguish patrilineal relatives because of low mutation rates. Rapidly mutating (RM) Y-STRs offer improved resolution, although population-specific reference data remain scarce. AIM: To evaluate the mutation rates and male differentiation capacity of 26 RM Y-STR loci in a British Isles population. METHODS: A total of 160 father-son pairs and three monozygotic twin pairs were analysed using the Microreader 26 RM-Yplex ID and Y Prime Plus ID Systems. Mutation rates were estimated using a frequentist approach, and differentiation capacity was compared between RM and conventional Y-STRs. RESULTS: RM Y-STRs differentiated 43.78% of father-son pairs compared with 10.01% using conventional Y-STRs. Both marker sets achieved complete discrimination among unrelated males, while RM Y-STRs increased the minimum difference between the closest haplotypes from three to 15 loci. Additional analyses demonstrated associations between mutation occurrence, allele size, and paternal age, and identified population-specific variant alleles that have implications for forensic interpretation. CONCLUSION: RM Y-STRs substantially improve the differentiation of patrilineal relatives and enhance haplotype resolution in a British Isles population. These findings support their forensic application while emphasising the need for larger population-specific datasets to strengthen mutation rate estimation and forensic interpretation.