Considerations for the interpretation of STR results in cases of questioned half-sibship

Development of a new 25plex STRs typing system for forensic application

We have developed a novel STR 25-plex florescence multiplex-STR kit (DNATyper25) to genotype 23 autosomal and two sex-linked loci for forensic applications and paternity analysis. Of the 23 autosomal loci, 20 are non-CODIS. The sex-linked markers include a Y-STR locus (DYS391) and the Amelogenin gene. We present developmental validation studies to show that the DNATyper25 kit is reproducible, accurate, sensitive, and robust. Sensitivity testing showed that full profiles were achieved with as low as 125 pg of human DNA. Specificity testing demonstrated a lack of cross reactivity with a variety of commonly encountered non-human DNA contaminants. Stability testing showed that full profiles were obtained with humic acid concentration ≤60 ng/μL and hematin concentration <400 μM. For forensic evaluation, the 23 autosomal STRs followed the Hardy-Weinberg equilibrium. In an analysis of 509 Chinese (CN) Hans, we detected a combined total of 181 alleles at the 23 autosomal STR loci. Since these autosomal STRs are independent from one another, PM was 8.4528 × 10^-22 , TDP was 0.999 999 999 999 999 999 999, CEP was 0.999 999 8395. The forensic efficiency parameters demonstrated that these autosomal STRs are highly polymorphic and informative in the Han population of China. We performed population comparisons and showed that the Northern CN Han has a close genetic relationship with the Luzhou Han, Tujia, and Bai populations. We propose that the DNATyper25 kit will be useful for cases where paternity analysis is difficult and for situations where DNA samples are limited in quantity and low in quality.

A simple method for establishing concordance between short-tandem-repeat allele frequency databases

BACKGROUND

Current paternity and forensic accreditation standards do not require concordance to be established between short-tandem-repeat allele frequency databases representing the same population. The current statistical methods for evaluating databases do not establish concordance. Although acceptable under current forensic statistical methods, databases representing the same population may have sufficient variation to influence the outcome of a nondirect relationship testing result (i.e., siblingship). Hence there is a need for a quantitative method to determine concordance between databases.

STUDY DESIGN AND METHODS

Local allele frequency databases were generated for major US ethnic groups. Statistical analysis was performed as recommended by international forensic standards. A new method was developed and used for evaluating concordance between the locally developed and published databases. Smaller deviation values signify greater concordance between compared databases. The results were quantitatively confirmed against data obtained from a multidimensional scaling analysis system (SPSS, SPSS, Inc.).

RESULTS

The locally developed database deviated from the mean of the published data by approximately 0.1073 for Caucasians, 0.1341 for US Hispanics, and 0.1287 for African Americans. Upon pairwise comparison, the published databases deviated from one another by 0.1210 in US Caucasians, 0.1457 in US Hispanics, and 0.1228 in African Americans. The local database was observed to be more concordant to published databases than some of the published data to themselves.

CONCLUSION

The developed database was shown to be concordant with four previously published databases using the new method. The results were confirmed by comparison to qualitative data obtained from the multidimensional scaling analysis system.

Efficacy of extended kinship analyses utilizing commercial STR kit in establishing personal identification

Unprecedented fidelity and specificity have afforded DNA testing its long reigning status as the gold standard for establishing personal identification. While the method itself is flawless, forensic experts have undoubtedly stumbled across challenging cases in which no reference samples for an unknown person (UP) are available for comparison. In such cases, experts often must resort to an assortment of kinship analyses-primarily those involving alleged parents or children of a UP-to establish personal identification. The present study derives likelihood ratio (LR) distributions from an extensive series of kinship simulations and places actual data, obtained from 120 cases in which personal identification of a UP was established via kinship analyses, to a comprehensive comparison in order to evaluate the efficacy of kinship assessments in establishing personal identification. A commercially available AmpFlSTR Identifiler kit was used to obtain DNA profiles. UP DNAs were extracted and isolated from fingernail (n=87), cardiac blood (24), carpal bone (7) and tooth (2). Buccal cells were procured from alleged kin (AK) for subsequent kinship analyses. In 72 cases 1-3 alleged children were available for comparison; in 46 cases, one or both alleged parents were available; and in the final 2 cases (involving a pair of bodies discovered together in a dwelling), their alleged children were typed for comparison. For each case a LR was calculated based on the DNA typing results. Interestingly, we found that the median LR observed in the actual cases virtually mirrored those of the simulations. With exception to 2 cases in which a silent allele was observed at D19S433, biological relatives showed a LR greater than 100 and in these cases, kinship between the UP and AK were further supported by additional forms of evidence. We show here that in the vast majority of identification cases where direct reference samples are unavailable for a UP, kinship analyses referring to alleged parents/children and using 15 standard loci is more than capable of establishing the identification of a UP. However, discretion should be advised for silent alleles which-albeit rare-are known to occur at loci such as D19S433, along with other mutations which could render a deceivingly reduced LR.