ABC Forensic DNA key terms

A tunable or filtered lamp used to search items for stains that fluoresce or absorb light, helping examiners locate biological evidence before sampling.

A rapid, sensitive screening assay that suggests a biological fluid or cell type is present but does not prove identity; positive results guide confirmatory testing and sampling.

A specific laboratory method that verifies the identity of a biological fluid or cell type, such as prostate-specific antigen for semen or sperm microscopy, before DNA analysis proceeds.

A sample taken from an unstained area of the same item or material as the evidence stain, processed alongside it to detect background DNA or reagent contamination.

DNA deposited by skin contact, often in very small amounts and mixed with background DNA, requiring careful collection, extraction, and interpretation to avoid misleading results.

A sample containing very few amplifiable DNA molecules, commonly below about 100 picograms, where stochastic effects such as allelic drop-out and drop-in become likely.

A sequential lysis procedure that separates spermatozoa from other cells in sexual assault samples, producing a sperm fraction enriched for male DNA and an epithelial fraction.

A solution containing detergents, salts, and buffers that disrupts cell membranes and releases DNA, often with proteinase K added to digest proteins during extraction.

A broad-specificity protease used during DNA extraction to digest cellular proteins and nucleases, improving DNA yield and purity while protecting nucleic acids from degradation.

A reducing agent that breaks disulfide bonds in keratin and sperm protein coats, enabling efficient lysis and DNA release from spermatozoa and other tough tissues.

A purification method in which DNA binds to silica membranes or beads in the presence of chaotropic salts, is washed, and then eluted in a low-salt buffer.

A substance that interferes with extraction, amplification, or detection, such as humic acids, hematin, indigo dyes, or collagen, potentially causing failed or weak profiles.

A PCR method that measures amplified product during each cycle, allowing estimation of human DNA concentration and sometimes degradation or male DNA content in a sample.

A known template added to each amplification reaction to monitor for inhibitors and reaction failure, distinguishing a negative sample from an unsuccessful test.

A single amplification reaction containing multiple primer pairs that simultaneously amplify several STR loci, conserving sample and time while producing a combined profile.

Repeated cycles of denaturation, primer annealing, and extension that copy targeted DNA sequences exponentially, enabling STR analysis from small or degraded samples.

A separation technique that moves fluorescently labeled DNA fragments through a narrow polymer-filled capillary under voltage, separating them by size for laser detection.

A graphical output from capillary electrophoresis showing fluorescence peaks by size or scan number, from which alleles, peak heights, and artifact patterns are interpreted.

A mixture of known alleles for every targeted STR locus, run with samples to calibrate fragment sizing and assign allele numbers by comparison.

A set of labeled DNA fragments of known sizes added to each capillary injection, used to convert scan data into fragment sizes and align samples with the allelic ladder.

A procedure using fluorescent dyes or matrix standards to correct overlap among detection channels, ensuring that each dye's signal is accurately separated in the electropherogram.

A minor peak one repeat unit smaller than the true allele, caused by polymerase slippage during amplification, commonly evaluated when interpreting mixtures and low-level peaks.

A sex-typing marker on the X and Y chromosomes that produces different-sized fragments, commonly included in STR multiplexes to indicate the presence of X and Y DNA.

One of the possible variants of a short tandem repeat locus, defined by the number of repeat units and typically named by its length in repeat units.

The pair of alleles detected at a genetic locus in an individual sample, which may be homozygous, heterozygous, or show additional alleles from mixtures or artifacts.

A genotype with two different alleles at a locus, producing two peaks that may differ in height because of sequence, size, or amplification effects.

The fluorescence intensity of an allele peak measured in relative fluorescence units, used to assess signal strength, genotype balance, mixture proportions, and stochastic thresholds.

The minimum peak height at which a signal is considered a reliable allele rather than noise, established during validation for each laboratory's detection system.

A peak height above which allelic drop-out at a heterozygous locus is unlikely, used to decide whether a profile is suitable for comparison or requires probabilistic interpretation.

Failure to detect an allele that is present in the sample, often because of low template quantity, degradation, inhibition, or poor amplification, leading to incomplete profiles.

A DNA profile containing alleles from two or more contributors, recognized by extra peaks, unbalanced heterozygotes, or allele counts exceeding the expected maximum at a locus.

A statistical measure comparing the probability of the evidence under two hypotheses, such as prosecution and defense, with values above one supporting the first hypothesis.

The probability that a randomly selected unrelated person from a reference population would have a DNA profile matching the evidence profile by chance.

A computational approach that models peak heights, drop-out, drop-in, and other stochastic effects to evaluate mixtures and calculate likelihood ratios under alternative hypotheses.

Planned activities and policies designed to ensure that laboratory processes, personnel, and results meet defined standards, including audits, training, validation, and document control.

Routine checks performed alongside casework, such as positive and negative controls, to verify that reagents, instruments, and procedures are working within acceptable limits.

A documented process demonstrating that a method, instrument, or software performs as intended for its forensic use, including sensitivity, specificity, reproducibility, and mixture studies.

A sample containing no target DNA, processed through extraction and amplification to detect contamination or reagent problems; it should produce no alleles.

Introduction of exogenous DNA or amplified product into a sample or laboratory area, which can create false alleles, mixtures, or misleading associations.

A documented, chronological record of who handled evidence, when, and for what purpose, preserving integrity and admissibility from collection through analysis and storage.