Taking account of peak areas when interpreting mixed DNA profiles

This paper establishes a logical framework for taking account of peak areas when interpreting mixed DNA STR profiles. The principles apply wherever such data are available but they are illustrated here by means of data which have been collected from made up mixtures of known concentrations analyzed at short tandem repeat loci. The data have led to some modeling assumptions which are used for numerical examples. In actual casework the proportions of the various components will not be known and there is a discussion of whether they should be allowed for by integrating over a prior distribution. This is a conceptual paper, rather than a prescription for casework, and the scope for further work is outlined.

Analysis of multiplexed short tandem repeat (STR) systems using capillary array electrophoresis

The profiling of polymorphic short tandem repeat (STR) markers is being applied to human identification, parentage testing and genetic mapping. Reliable genotyping of these markers is facilitated by polymerase chain reaction (PCR) amplification and high-resolution electrophoretic separation. Capillary array electrophoresis (CAE) offers very rapid, high-resolution separation of the amplified DNA and potential for automated sample processing not realized employing conventional slab-gel electrophoresis. The use of CAE to type DNA samples amplified at 11 genetic loci in multiplex profiles is presented. Two sets totaling 208 samples were amplified in a multiplex fashion using AmpFlSTR-Blue or AmpFlSTR-Green I and analyzed in a blind study using CAE. With the exception of one sample, the CAE genotyping results were in complete agreement with results obtained using a single-capillary system or two slab-gel electrophoresis systems. The sample, genotype TH01 7/10, migrated similar to TH01 6.3/9.3 allele sizes, which suggested a potential band migration shift. The recommended approach to such an observation is to analyze the sample again. The sample was rerun and correct genotype verified. Allelic ladder samples were analyzed multiple times by CAE to determine sizing accuracy and precision. The sizing of over 240 allelic ladder samples yielded an average within-run precision of +/- 0.13 bp and between-run precision of +/- 0.21 bp for fragments up to 350 bp. The CAE protocols permit processing of up to 96 multiplex STR samples in under 70 min.

Statistical analysis of data for three British ethnic groups from a new STR multiplex

Data have been collected from 602 Caucasians, 190 Afro-Caribbeans and 257 Asians of Indo/Pakistani descent who have been profiled using a new six locus short tandem repeat (STR) multiplex. The data have been analysed by conventional significance testing methods: the exact test, homozygosity, and conventional goodness of fit to Hardy-Weinberg proportions. Frequency tables are given and the expected performance in British forensic casework is discussed.

Validation of the Applied Biosystems Prism 377 automated sequencer for the forensic short tandem repeat analysis

The Applied Biosystems (ABI) Prism 377 DNA sequencer has been evaluated in an attempt to increase the throughput of samples for short tandem repeat (STR) analysis, in both forensic casework and the UK National Criminal Intelligence DNA Database. The gel system assessed consisted of 0.2 mm, 4% acrylamide 6 M urea gels, with a well-to-read distance of 36 cm. Gels were run at a constant voltage of 3 kV and constant temperature of 51 degrees C. The run time of our second generation multiplex (SGM) STR system was achieved in less than 2 h. Rigorous validation has been performed on the instrument hardware and software. Complete resolution of 1 base differences was obtained, up to and beyond 350 bases; sizing precision across gels was more than 2-fold higher than the 373A and the sensitivity was increased by one third.

Establishing the robustness of short-tandem-repeat statistics for forensic applications

Before the introduction of a four-locus multiplex short-tandem-repeat (STR) system into casework, an extensive series of tests were carried out to determine robust procedures for assessing the evidential value of a match between crime and suspect samples. Twelve databases were analyzed from the three main ethnic groups encountered in casework in the United Kingdom: Caucasians, Afro-Caribbeans, and Asians from the Indian subcontinent. Independence tests resulted in a number of significant results, and the impact that these might have on forensic casework was investigated. It is demonstrated that previously published methods provide a simple procedure for correcting allele frequencies--and that this leads to conservative casework estimates of evidential value.

A highly discriminating octoplex short tandem repeat polymerase chain reaction system suitable for human individual identification

Through the use of fluorescence-based polymerase chain reaction systems, a highly discriminating multiplex with the potential for individual identification has been developed. The use of multiple dye technology enabling loci with overlapping size ranges to be co-amplified has enabled us to successfully amplify seven tetranucleotide short tandem repeat loci within a single reaction resulting in a discriminating power in the region of 1 x 10(9). Three out of the seven loci employed exhibit alleles differing in size by only 2 bp as opposed to the conventional 4 bp, which results in such loci being more powerful in terms of distinguishing between samples, particularly when co-amplified in this manner. The size ranges of the loci contained within the system are such that windows still exist for the inclusion of additional loci at a later stage, which could increase the discriminating power of the system still further. In addition, further weight and utility is lent to the system through the incorporation of a simple and reliable sex test involving the amplification of a segment of the X-Y homologous gene Amelogenin.

Survey of isoenzymes in the snail Cepaea nemoralis using different buffer/gel systems in polyacrylamide disc gel electrophoresis: validity of comparisons and effect of "nothing dehydrogenase" activity

An investigation into isoenzymic analysis using four different buffer/gel systems was carried out. Fourteen different isoenzyme systems were surveyed on each buffer/gel system. It is shown that comparable results are not obtained between different systems. Previous workers in Cepaea nemoralis have used different buffer/gel systems from one another. Alkaline phosphatase and acid phosphatase and certain lactate and malate dehydrogenases are shown to behave similarly in three different systems. It is tentatively suggested that these isoenzymes may be genetically identical. "Nothing dehydrogenase" activity is demonstrated in gels stained for other dehydrogenases. It is suggested that nothing dehydrogenase activity may be attributable to glutamate or malate dehydrogenase.