An overall limited effect on the weight-of-evidence when taking STR DNA sequence polymorphism into account in kinship analysis

Abstract

The recent year’s development of massively parallel sequencing (MPS) instruments and assays have now made it a compatible complement to the established capillary electrophoresis (CE) analysis for different forensic genetic applications. It is well known that short tandem repeat (STR) alleles of the same fragment size could have different DNA sequences. Thus, there will be an expected increase in the population genetic diversity for the present set of forensic STRs when performing the analysis with MPS technologies and taking the internal DNA sequence into account. In order to study the additional value of this increase of information for kinship analysis casework, we set up an allele frequency database for the Swedish population for the autosomal markers included in the ForenSeq™ DNA Signature Prep Kit (Illumina). A total of 298 individuals with Swedish origin were analyzed and allele frequency distributions based on DNA sequence polymorphisms for 27 autosomal STRs were established. As expected, the results showed an addition in number of observed alleles with 55% in total compared with fragment length based allele definitions, however, a majority only appeared in a few number of observations. In addition, simulations were performed in order to study the impact of the increase in number of observed alleles for the expected likelihood ratios (LRs) for different kinship case scenarios. Only a minor increase of the LRs were, however, observed when taking allele sequence variations in addition with fragment length variations into account compared to only considering fragment length variations. Further studies are required to see if it is cost effective to implement this technique that, according to this study, only has a limited overall additive effect for kinship testing. Although, in specific cases MPS methods will increase the discrimination power due to that, even if in a low frequency, a high genetic diversity exist and the differentiation could be more significant. The establishment of the allele frequency database will enable biostatistical calculations to be performed in casework.