4 research outputs found
Estimating the number of contributors to two-, three-, and four-person mixtures containing DNA in high template and low template amounts
Aim To develop guidelines to estimate the number of contributors
to two-, three-, and four-person mixtures containing
either high template DNA (HT-DNA) or low template
DNA (LT-DNA) amounts.
Methods Seven hundred and twenty-eight purposeful
two-, three-, and four-person mixtures composed of 85
individuals of various ethnicities with template amounts
ranging from 10 to 500 pg were examined. The number
of alleles labeled at each locus and the number of labeled
different and repeating alleles at each locus as well over all
loci for 2 HT-DNA or 3 LT-DNA replicates were determined.
Guidelines based on these data were then evaluated with
117 mixtures generated from items handled by known individuals.
Results The number of different alleles over all loci and
replicates was used to initially categorize mixtures. Ranges
were established based on the averages plus and minus
2 standard deviations, and to encompass all observations,
the maximum and the minimum values. To differentiate
samples that could be classified in more than one grouping,
the number of loci with 4 or more repeating or different
alleles, which were specific to three- and four-person
mixtures, were verified. Misclassified samples showed an
extraordinary amount of allele sharing or stutter.
Conclusions These guidelines proved to be useful tools to
distinguish low template and high template two-, three-,
and four-person mixtures. Due to the inherent higher
probability of allele sharing, four-person mixtures were
more challenging. Because of allelic drop-out, this was also
the case for samples with very low amounts of template
DNA or extreme mixture ratios
Estimating the number of contributors to two-, three-, and four-person mixtures containing DNA in high template and low template amounts
Aim To develop guidelines to estimate the number of contributors
to two-, three-, and four-person mixtures containing
either high template DNA (HT-DNA) or low template
DNA (LT-DNA) amounts.
Methods Seven hundred and twenty-eight purposeful
two-, three-, and four-person mixtures composed of 85
individuals of various ethnicities with template amounts
ranging from 10 to 500 pg were examined. The number
of alleles labeled at each locus and the number of labeled
different and repeating alleles at each locus as well over all
loci for 2 HT-DNA or 3 LT-DNA replicates were determined.
Guidelines based on these data were then evaluated with
117 mixtures generated from items handled by known individuals.
Results The number of different alleles over all loci and
replicates was used to initially categorize mixtures. Ranges
were established based on the averages plus and minus
2 standard deviations, and to encompass all observations,
the maximum and the minimum values. To differentiate
samples that could be classified in more than one grouping,
the number of loci with 4 or more repeating or different
alleles, which were specific to three- and four-person
mixtures, were verified. Misclassified samples showed an
extraordinary amount of allele sharing or stutter.
Conclusions These guidelines proved to be useful tools to
distinguish low template and high template two-, three-,
and four-person mixtures. Due to the inherent higher
probability of allele sharing, four-person mixtures were
more challenging. Because of allelic drop-out, this was also
the case for samples with very low amounts of template
DNA or extreme mixture ratios
Validation of Testing and Interpretation Protocols for Low Template DNA Samples Using AmpFℓSTR® Identifiler®
Aim To test the reliability, robustness, and reproducibility
of short tandem repeat (STR) profiling of low template
DNA (LT-DNA) when employing a defined set of testing
and interpretation parameters.
Methods DNA from known donors was measured with
a quantitative real time polymerase chain reaction (PCR)
assay that consistently detects less than 1 pg/μL of DNA
within a factor of 0.3. Extracts were amplified in triplicate
with AmpFSTR® Identifiler® reagents under enhanced PCR
conditions. Replicates were examined independently and
alleles confirmed using a consensus approach. Considering
observed stochastic effects inherent to LT-DNA samples,
interpretation protocols were developed and their accuracy
verified through examination of over 800 samples.
Results Amplification of 100 pg or less of DNA generated
reproducible results with anticipated stochastic effects.
Down to 25 pg of DNA, 92% or more of the expected alleles
were consistently detected while lower amounts
yielded concordant partial profiles. Although spurious alleles
were sometimes observed within sample replicates,
they did not repeat. To account for allelic dropout, interpretation
guidelines were made especially stringent for
determining homozygous alleles. Due to increased heterozygote
imbalance, stutter filters were set conservatively
and minor components of mixtures could not be resolved.
Applying the resultant interpretation protocols, 100% accurate
allelic assignments for over 107 non-probative casework
samples, and subsequently 319 forensic casework
samples, were generated.
Conclusion Using the protocols and interpretation guidelines
described here, LT-DNA testing is reliable and robust.
Implementation of this method, or one that is suitably verified,
in conjunction with an appropriate quality control
program ensures that LT-DNA testing is suitable for forensic
purposes