ABC Forensic DNA flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Kastle-Meyer test

Presumptive blood test in which hemoglobin catalyzes oxidation of phenolphthalein by hydrogen peroxide, producing a pink color; plant peroxidases can cause false positives.

Acid phosphatase test

Presumptive semen test based on high acid phosphatase activity in seminal fluid; a color change suggests semen, but other body fluids and plant materials can produce false positives.

Differential extraction

Extraction method that first lyses epithelial cells, then digests sperm heads with DTT and proteinase K, producing separate non-sperm and sperm DNA fractions.

Alternate light source

Screening tool using specific wavelengths and filter goggles to visualize latent biological stains such as semen or saliva; fluorescent areas still require presumptive and confirmatory testing.

PSA or p30 test

Confirmatory semen test detecting prostate-specific antigen, a protein from prostate epithelial cells; it is more specific than acid phosphatase but can occur at low levels in other male fluids.

Sperm microscopy

Confirmatory semen method in which a stained smear is examined for sperm heads with tails; absence does not exclude azoospermic or vasectomized donors.

Chelex extraction

Rapid extraction method using Chelex resin to bind metal ions and protect DNA during boiling; the product is single-stranded DNA suitable for PCR-based typing.

DTT in extraction

Dithiothreitol reduces disulfide bonds in sperm protamines, allowing proteinase K to digest sperm heads; it is added after epithelial cells are lysed in differential extraction.

qPCR quantitation

Real-time PCR assay that measures human DNA and can estimate male DNA, degradation, and inhibition; it is human-specific and more informative than fluorometry for forensic samples.

Degradation index

Ratio of qPCR amplification of a long target to a short target; values above a validated cutoff indicate DNA fragmentation and possible stochastic amplification effects.

Internal PCR control

Spike added to qPCR reactions to monitor inhibition; failure or delayed amplification indicates inhibitors that may require dilution or cleanup before STR typing.

PCR multiplex

Simultaneous amplification of many STR loci using dye-labeled primers and optimized buffer; multiplex kits include allelic ladders and size standards for typing.

Allelic ladder

Reference mixture containing common alleles at each STR locus; it is run with samples to calibrate fragment sizing and assign allele numbers by comparison.

Internal size standard

Fluorescently labeled DNA fragments of known sizes added to each capillary electrophoresis injection; they create a size curve converting migration times to base-pair sizes.

Amelogenin marker

Sex-typing marker with different X and Y fragment sizes; it usually shows female as X,X and male as X,Y, but a Y deletion can produce a female-like result.

Stutter peak

Artifact peak usually one repeat unit shorter than the true allele, caused by slippage during PCR; laboratories apply locus-specific stutter filters before interpretation.

Pull-up artifact

Capillary electrophoresis artifact in which a strong signal in one dye channel appears as a smaller peak in another channel; spectral calibration helps prevent it.

Off-ladder allele

STR allele whose size does not match a ladder allele within the validated bin; it may be a rare variant and requires confirmation and documentation.

Analytical threshold

Peak height or area threshold below which capillary electrophoresis signal is not reliably distinguished from noise; peaks below it are not interpreted as alleles.

Random match probability

Probability that a randomly selected unrelated person has the same DNA profile; it is estimated with the product rule using population allele frequencies.

Likelihood ratio

Ratio of the probability of the DNA evidence under the prosecution hypothesis to its probability under the defense hypothesis; it quantifies support for one proposition.

Combined probability of inclusion

For a diploid mixture, the product across loci of the squared sum of allele frequencies; it estimates the chance a random person would be included as a contributor.

Allelic drop-out

Failure to detect an allele that is present because its peak falls below the analytical threshold, often from low template DNA; it can make a heterozygote appear homozygous.

Allelic drop-in

Spurious allele peak not derived from the sample, usually seen with low template DNA; replicate amplifications and threshold rules help distinguish it from true alleles.

Probabilistic genotyping

Software-based mixture interpretation using peak heights, drop-out probabilities, and statistical models; it outputs a likelihood ratio rather than a simple inclusion or exclusion.

Theta correction

Population substructure adjustment using Fst in genotype probability calculations; it accounts for relatedness within populations and generally makes match probability estimates less extreme.

Reagent blank

Negative extraction control containing reagents but no sample; it monitors contamination and must produce no DNA profile or remain below validated thresholds.

Negative amplification control

PCR reaction with water or buffer instead of DNA; it detects contamination in reagents or workflow and should yield no alleles.

Proficiency test

Test sample with known or unknown profile used to evaluate analyst performance; results are reviewed for compliance and may be required for accreditation.

Internal validation

Laboratory study before casework demonstrating a method's reliability, sensitivity, specificity, and thresholds under local conditions; results support standard operating procedures.

Chain of custody

Chronological documentation of evidence transfer, storage, and handling; it maintains integrity and supports admissibility by showing who had the item and when.

CODIS

Combined DNA Index System; FBI-administered network of local, state, and national databases used to search eligible DNA profiles for investigative leads.