| In the field of public security,forensic physical evidence identification of mixed biological evidence at crime scenes is a difficult and focal point for technical researchers worldwide.At present,forensic DNA evidence mainly applies autosomal Short Tandem Repeat（STR）test technology for individual identification and relative identification.However,it is difficult to disentangle the mixed DNA extracted from biological evidence at scenes by relying on a single autosomal STR technique,therefore,it is of great importance to develop and establish a mixed biological evidence examination system with high sensitivity,wide applicability and compatibility with existing forensic evidence examination platforms to enhance the discrimination and resolution of mixed DNA samples.It was shown that DIP-STR markers can target amplification of minor components in mixed DNA by linking deletion/insertion polymorphic sites（DIP）to STR.DIP-STR is not only highly polymorphic,but also has the same typing method as conventional STR analysis,which is easy to be used by forensic laboratories.To further develop marker loci suitable for Chinese population,this paper relied on the forensic evidence capillary electrophoresis sequencing platform to screen and validate nine novel DIP-STR loci suitable for Chinese population and evaluate the system’s ability to identify mixed DNA.The main elements are as follows:1.9 novel unreported DIP-STR locis were screened and validated,and the amplification efficiency of the designed primers was evaluated by combining Sanger sequencing and capillary fluorescence electrophoresis platforms.The detection threshold of the screened DIP-STR loci could be as low as 0.005 ng,and the mixed sample test could detect the minor donor typing in the mixed ratio of 1000:1 at the lowest.2.Forensic genetic parameters analysis of the Chinese population for the newly screened DIP-STR locis was performed.DNA samples from 200 blood cards of unrelated individuals were extracted and the forensic genetic parameters of all 12 locis,including the3 locis identified in the previous study,were evaluated as a whole.The study showed that the comprehensive discriminatory power of the system reached 0.9999999997,with a random matching probability of 2.79×10-10 and an average informative genotype frequency of 0.363.This identification system can meet the needs of individual identification to a certain extent and play a key role in the identification of secondary donors in the analysis of mixed biological evidence.3.A calibration model for informative genotype frequencies was established.Machine learning calibration models were constructed using linear regression analysis,support vector machine regression and regression tree analysis,respectively.Among them,the calibration results of the regression tree analysis model are the most accurate and can comprehensively consider the influence of observed heterozygosity,which makes the calculated values of informative genotype frequencies more objective and provides guidance for the subsequent development and evaluation of loci.4.The DIP-STR test system and data analysis method based on the forensic NGS platform were initially established.Based on the experimental results of capillary fluorescence electrophoresis platform,2 DIP-STR locis were selected for library preparation and sequencing on NGS platform,and the typing results consistent with capillary fluorescence electrophoresis platform were obtained.5.Trial of DIP-STR test method in practical cases.Using the DIP-STR test system and method established in this study,the mixed biological evidence in the case was tried and compared with the detection results of conventional STR kits.The results show that DIP-STR has significant advantages in the unbalanced mixed biological evidence testing.Meanwhile,the likelihood ratio can be calculated to assess the individual identification ability of the current DIP-STR system,which can provide scientific basis for test identification and court proceedings. |
PDF Full Text Request