<p class="first" id="d5860482e110">The discovery of rapidly mutating (RM) Y-STRs started
to move the field of forensic
Y-STR analysis from male lineage identification towards male individual identification.
Previously, the forensic value of RM Y-STRs for differentiating male relatives was
limited due to the modest number of 13 identified RM Y-STRs. Recently, new RM Y-STRs
were discovered, with strong expectations for significantly improving male relative
differentiation; however, empirical evidence is missing yet. More recently, the genotyping
method RMplex for efficiently analyzing 30 Y-STRs with high mutation rates, including
all 26 currently known RM Y-STRs, was introduced. Here, we applied RMplex as well
as the current state-of-the-art commercial Y-STR kit: Yfiler™ Plus PCR Amplification
kit, to several hundreds of DNA-confirmed father-son pairs. Newly established estimates
confirmed the high mutation rates of novel and previous RM Y-STRs. By combining current
with previous data, we provide updated consensus estimates of mutation rates for all
49 Y-STRs targeted with both methods. Based on RMplex, 42% of 499 father-son pairs
were differentiated, while 14% of 530 pairs based on Yfiler™ Plus, and 48% of 499
pairs based on both methods combined. Regarding brothers, RMplex also clearly outperformed
Yfiler™ Plus, with differentiation rates of 62% and 33%, respectively. By combining
both methods 72.9% of the brothers showed at least one mutation. For unrelated males,
both methods achieved a discrimination capacity of 99.8% and a haplotype diversity
of 0.999991, since all males had different haplotypes, except for two, perhaps indicating
a hidden paternal relationship. Overall, this study underlines the value of RM Y-STRs
in general and RMplex in particular for differentiating male relatives highly relevant
in forensic genetics. It provides the first empirical evidence on the high value of
RMplex for differentiating close male relatives, which for father-son pairs was almost
60% higher than with the initial set of 13 RM Y-STRs and three times higher than with
Yfiler™ Plus. Based on our results from closely related males, we expect RMplex to
also improve the differentiation of more distantly related males significantly, which
needs empirical demonstration in future studies. We encourage the forensic community
to apply RMplex in all forensic cases where a match with a commercial Y-STR kit was
obtained between the male suspect and the evidence material, or to solely use RMplex
in such cases, aiming to find out if the male suspect or any of his male paternal
relatives left the evidence material at the crime scene.
</p>