Glossary of forensic medicine

3’ (“three prime”): refers to the end of a DNA molecule bearing a free hydroxyl group on the 3’ carbon of the sugar ring
5’ (“five prime”): refers to the end of a DNA molecule bearing a free hydroxyl group on the 5’ carbon of the sugar ring; a DNA sequence is typically read from the 5’ to 3’ end
9947A: a cell line derived from a human female; DNA from this cell line is used as a positive control in some commercial DNA test kits
ABFO scales (American Board of Forensic Odontology scales). An L-shaped piece of plastic used in photography that is marked with circles, black and white bars, and 18-percent gray bars to assist in distortion compensation and provide exposure determination. For measurement, the plastic piece is marked in millimeters.
ABI Genetic Analyzer: a capillary electrophoresis instrument sold by Applied Biosystems and widely used throughout the forensic DNA community since its introduction in 1995; an ABI 310 uses a single capillary filled with a viscous polymer solution to separate DNA molecules based on their relative size; laboratories often use a multi- capillary instrument such as the ABI 3130 (4-capillaries), ABI 3130xl (16-capillaries), ABI 3500 (8-capillaries), or ABI 3500xl (24-capillaries) Genetic Analyzer
ABI: Applied Biosystems Inc.; an instrument and reagent manufacturing company based in South San Francisco, California that is now called ThermoFisher Scientific
Accreditation: the formal process by which a laboratory is evaluated, with respect to established criteria, for its competence to perform a specified kind of measurement; the evaluation takes place through an audit by an outside, qualified agency and helps provides confidence that the laboratory meets minimum professional standards for general operations
Accuracy: the degree of agreement or conformity of a measured value with its actual (true) value
Adventitious DNA: contaminating DNA that unintentionally becomes amplified along with the intentional template DNA obtained from evidence or reference samples
Adventitious match: an association of an evidence DNA profile to the profile of a person who is not the true donor of that profile; this situation may arise when the DNA profile contains information from a limited number of loci (e.g., from a damaged DNA sample) that are insufficient to distinguish the profiles of two different individuals
Alle lic ladde r: in STR testing, a measurement calibration tool, consisting of the most commonly observed alleles, used for assigning an allele designation to a peak in an electropherogram at a particular genetic locus
Allele drop-in: allelic peak(s) in an electropherogram that are not reproducible across multiple independent amplification events; also a hypothesis/postulate for the observation of one or more allelic peaks in an electropherogram that are inconsistent with the assumed/known contributor(s) to a sample
Allele drop-out: failure of an otherwise amplifiable allele to produce a signal above the analytical threshold because the allele was not present or was not present in sufficient quantity in the aliquot that underwent PCR amplification
Allele frequency: the number of times that an allele appears in a data set is the absolute frequency; the proportion of a particular allele among the chromosomes carried by individuals in a population
Allele: one of two or more versions of a genetic sequence at a particular location (a locus) in the genome; typically, multiple alleles are possible with STR markers
Alternate light source Equipment used to produce visible and invisible light at various wavelengths to enhance or visualize potential items of evidence (fluids, fingerprints, clothing fibers, etc.).
Amelogenin: a commonly used sex-typing genetic marker because the gene, which codes for tooth enamel, occurs on both the X and Y chromosomes; the X-chromosome region amplified in commercial STR kits is 6 bases shorter than the Y-chromosome region such that female (X,X) samples result in a single peak while males (X.Y) have two peaks
Amplicon: the product of polymerase chain reaction DNA amplification
Amplification: an increase in the number of copies of a specific DNA fragment; can be in vivo or in vitro; in forensic DNA testing laboratories, this refers to the use of the PCR technique to produce many more copies of DNA (typically alleles) at specific genetic loci
Analyst: an individual trained and qualified to report DNA results
Analytical threshold: a RFU level determined to be appropriate for use in the PCR/STR DNA typing process; a minimum threshold for data comparison that is identified in each laboratory through validation studies
Artifact: any non-allelic product of the amplification process (e.g., stutter or non-template addition), anomaly of the detection process (e.g., pull-up or spike), or a by-product of primer synthesis (e.g., dye blob) that may be observed in an electropherogram; may complicate interpretation of a DNA profile when they cannot be distinguished from actual allele(s) data
Audit: evaluation of a laboratory based on a set of criteria established by an outside, qualified agency usually for establishing accreditation
Autosome: a chromosome not involved in sex determination; the diploid human genome consists of 46 chromosomes, 22 pairs of autosomes, and one pair of sex chromosomes (the X and Y chromosomes).
Base pair (bp): two complementary nucleotides joined by hydrogen bonds; base pairing occurs between A and T and between G and C
Base sequence: the order of nucleotide bases in a DNA molecule; typically read from the 5’-end to the 3’-end
Bayesian approach: defines the probability of an event as the degree of belief in the truth of a proposition that asserts it will happen
Bin: a range of sizes established empirically that is used to establish allele designation for length-based differences in STR alleles
Bindle paper Clean paper folded to use to contain trace evidence, sometimes included as part of the packaging for collecting trace evidence.
Biohazard bag A container for materials that have been exposed to blood or other biological fluids and have the potential to be contaminated with hepatitis, AIDS, or other viruses.
Biological fluids Fluids that have human or animal origin, most commonly encountered at crime scenes (e.g., blood, mucus, perspiration, saliva, semen, vaginal fluid, urine).
Biological weapon Biological agents used to threaten human life (e.g., anthrax, smallpox, or any infectious disease).
Bloodborne pathogen Infectious, disease-causing microorganisms that may be found or transported in biological fluids.
Boundaries The perimeter or border surrounding potential physical evidence related to the crime.
Buccal swab: a relatively non-invasive technique of scrapping the inside of a mouth with a cotton swab or similar collection device to collect cells from the inner cheek lining; a common method for collecting and preserving samples for DNA testing from known individuals
CODIS loci: an established set of STR loci required for inclusion of a DNA profile at the national level in CODIS; originally the core set involved 13 STRs but after January 1, 2017, 20 core STRs will be required for entry at the national level
CODIS: Combined DNA Index System, which under the direction of the FBI Laboratory, is the software architecture that runs the U.S. national DNA database
Capillary e le ctrophoresis (CE): an electrophoretic technique for separating DNA molecules by their size based on migration through a narrow glass capillary tube filled with a liquid polymer
Case file The collection of documents comprising information concerning a particular investigation. (This collection may be kept in case jackets, file folders, ring binders, boxes, file drawers, file cabinets, or rooms. Sub-files are often used within case files to segregate and group interviews, media coverage, laboratory requests and reports, evidence documentation, photographs, videotapes, audiotapes, and other documents.)
Case identifiers The alphabetic and/or numeric characters assigned to identify a particular case.
Cell: the basic building block of an organism; humans have approximately 100 trillion cells in their body, most containing DNA
Chain of custody A process used to maintain and document the chronological history of the evidence. (Documents should include name or initials of the individual collecting the evidence, each person or entity subsequently having custody of it, dates the items were collected or transferred, agency and case number, victim's or suspect's name, and a brief description of the item.)
Chelex extraction: a method of removing DNA from cells involving Chelex resin; since one step of the method involves boiling, the extracted DNA is single-stranded
Chemical enhancement The use of chemicals that react with specific types of evidence (e.g., blood, semen, lead, fingerprints) in order to aid in the detection and/or documentation of evidence that may be difficult to see.
Chemical threat Compounds that may pose bodily harm if touched, ingested, inhaled, or ignited. These compounds may be encountered at a clandestine laboratory, or through a homemade bomb or tankard leakage (e.g., ether, alcohol, nitroglycerin, ammonium sulfate, red phosphorus, cleaning supplies, gasoline, or unlabeled chemicals).
Chromosome: the structure by which hereditary information is physically transmitted from one generation to the next
Clean/sanitize The process of removing biological and/or chemical contaminants from tools and/or equipment (e.g., using a mixture of 10-percent household bleach and water).
Cold hit: an association made between a crime scene DNA profile and a DNA profile found on a DNA database in the absence of any prior investigative leads; this association may be to another crime scene DNA profile or to a profile from a known individual
Collect/collection The process of detecting, documenting, or retaining physical evidence.
Combined Probability of Inclusion (CPI): the product of the probabilities of inclusion calculated for each locus; the probability of inclusion at each locus estimates the probability that a randomly selected, unrelated individual is not excluded from being a source of the DNA mixture profile and is calculated as the square of the sum of the relative frequencies of the observed alleles at the locus; sometimes referred to as Random Man Not Excluded (RMNE)
Comparison samples A generic term used to describe physical material/ evidence discovered at crime scenes that may be compared with samples from persons, tools, and physical locations. Comparison samples may be from either an unknown/questioned or a known source.
Complementary sequences: nucleic acid base sequences that form a double-stranded structure by matching base pairs; the complementary sequence to G-T-A-C is C-A-T-G
Complete profile: a full DNA result with values being obtained from all attempted loci
Complex mixture: DNA profile resulting from a sample containing three or more contributors often having only a small amount of DNA from at least one of the contributors
Concordance: obtaining the same value when testing multiple times
Consensus profile: the resulting DNA profile made up from the list of alleles that are repeated or reproduced a specified number of times when a DNA extract is divided into several replicate samples that are amplified concurrently
Conservative: an assignment of the weight of evidence that is believed to favor the defense
Contamination The unwanted transfer of material from another source to a piece of physical evidence.
Contamination: exogenous DNA or other biological material in a DNA sample, PCR reaction, or item of evidence; the exogenous DNA or biological material could be present before the sample is collected or introduced during collection or testing of the sample
Controls: samples of known types, run in parallel with experimental, reference, or evidence samples, that are used to demonstrate that a procedure is working correctly
Critical reagents: chemicals or other materials used in testing whose performance is vital to the success of the test as determined by empirical studies or routine practice
Cross-contamination The unwanted transfer of material between two or more sources of physical evidence.
DNA databank: a repository of stored bloodstain cards or DNA samples used to generate DNA profiles
DNA database: a computer repository of DNA profiles
DNA profile: a string of values (numbers or letters) compiled from the results of DNA testing at one or more genetic markers; can be single- source or a mixture from multiple contributors
DNA profiling or typing: any methodology for generating data from a biological sample at one or more DNA loci; DNA test results can be compared to results obtained from other samples
DNA sequence: the relative order of base pairs, whether in a fragment of DNA, a gene, a chromosome, or an entire genome
Deconvolution: separation of component DNA genotypes of contributors to a mixed DNA profile based on quantitative peak height information and any underlying assumptions (e.g., the number of contributors to the mixture, mixture ratios, or known contributors)
Deduced profile: inference of an unknown contributor’s DNA profile after taking into consideration the contribution of a known/assumed contributor’s DNA profile based on quantitative peak height information in a mixed DNA profile
Degradation: the fragmenting, or breakdown, of DNA by chemical, physical, or biological means; a common occurrence when biological samples containing DNA encounter warm moist environments or excessive UV light
Deoxyribonucleic acid (DNA): a genetic material of organisms, usually double-stranded; a biopolymer composed of nucleic acids, identified by the presence of deoxyribose, a sugar, and the four nucleobases; DNA is a fairly stable molecule and variations in DNA sequence between individuals permits DNA testing to distinguish individuals from one another
Detection limit: the smallest amount of some component of interest that can be measured by a single measurement with a stated level of confidence
Differential extraction: a DNA extraction procedure where the sperm cells are physically separated from the DNA of other cells before the sperm DNA is isolated; generally results in a sperm and non-sperm (epithelial) fraction
Diploid: having two copies of each autosome; the normal constitution of most mammalian somatic cells
Direct PCR: the process of amplifying the DNA present in biological samples without prior extraction or purification
Documentation Written notes, audio/videotapes, printed forms, sketches and/or photographs that form a detailed record of the scene, evidence recovered, and actions taken during the search of the crime scene.
Double helix: the native form of DNA, which looks like a twisted ladder; two linear strands of DNA assume this shape when held together by complementary base pairing (ladder rungs)
Double-stranded DNA: form of DNA in which the individual strands are held together by complementary base pairing
Dye blobs: artifact peaks in capillary electropherograms arising from fluorescent dye molecules coming off their associated PCR primer during primer synthesis (and failing to be purified); generally, dye blobs are lower in signal intensity, and can be characteristically sized in each color channel
Dying declaration Statements made by a person who believes he or she is about to die, concerning the cause or circumstance surrounding his or her impending death.
Electropherogram: graphic representation of the separation of molecules by electrophoresis in which data appear as “peaks” along a line; the format in which DNA typing results are presented with the X- axis displaying the observed alleles in order of increasing size and the Y-axis recording the relative amount of DNA detected based on the fluorescent signal collected during analysis
Electrophoresis: a technique in which molecules are separated by their velocity in an electric field
Elimination database: collection of DNA profiles held in a searchable format whose access/role/activities are deemed to be a potential DNA contamination risk; DNA profiles from an elimination database may be used to identify instances of inadvertent contamination
Elimination sample: a sample collected from an individual who had lawful access to the crime scene (e.g., the spouse of a rape victim, police investigators, laboratory staff); results from this sample may be helpful in deconvoluting a DNA mixture
Enhanced DNA detection methods: those steps or methods employed during or subsequent to the PCR amplification step that increases the sensitivity of the standard method and are typically employed with low- quantity and/or low-quality samples; may include increased number of PCR amplification cycles, increased CE injection time or voltage, or post-amplification desalting or concentration
Epithelial cells: skin cells, vaginal cells, or other cells that are normally found on an inner or outer body surface
Eukaryote: an organism with cells containing a nucleus
Evidence identifiers Tape, labels, containers, and string tags used to identify the evidence, the person collecting the evidence, the date the evidence was gathered, basic criminal offense information, and a brief description of the pertinent evidence.
Evidence sample: biological sample collected from a crime scene or people or objects associated with a crime scene; sometimes referred to as the “Q” or question sample
Exclusion: a conclusion statement that a biological sample did not originate from a particular source or individual
Exon: segment of a gene that is represented in the final mRNA product
Expert system: a software program or set of software programs that interprets the data generated from a DNA analysis instrument platform in accordance with laboratory-defined quality assurance rules and accurately identifies the data that does and does not satisfy such rules
FST: the between-person inbreeding coefficient used in subpopulation corrections; commonly equated with the theta (θ) correction
First responder(s) The initial responding law enforcement officer(s) and/or other public safety official(s) or service provider(s) arriving at the scene prior to the arrival of the investigator(s) in charge.
Fluorescence: the emission of light from a molecule following its excitation by light energy; in the context of STR DNA analysis, different fluorescent dyes permit simultaneous detection of similar size PCR products through fluorescence emission in different wavelengths (colors); the relative amount of light emitted is recorded on the vertical axis of an electropherogram in relative fluorescence units, RFUs
Forensic science: the application of scientific knowledge to questions of civil and criminal law, typically through presentation of results from evidence in court
Frequentist approach: defines the probability of an event as the frequency with which the event occurs in the long-run of a sequence of identical trials where the event may or may not occur
Gene: the basic unit of heredity; a sequence of DNA nucleotides on a chromosome passed from parents to offspring that specifies traits
Genetics: branch of biology that deals with the heredity and variation of organisms
Genome: the entire DNA sequence found in a cell; the human genome consists of approximately 3.1 billion base pairs of DNA sequence
Genotype: the genetic makeup of an organism, as characterized by its physical appearance or phenotype; with STR DNA testing, a lcous genotype generally consists of two alleles, inherited from one’s mother and father
GlobalFiler: a multiplex STR typing kit from Applied Biosystems that co-amplifies 21 autosomal STRs, DYS391, a Y indel, and the sex-typing marker amelogenin
Glossary of Forensic DNA Terms Definitions are from Butler, J.M. (2010) Fundamentals of Forensic DNA Typing. San Diego: Elsevier Academic Press and other sources.
Glossary of Forensic DNA Terms Definitions are from Butler, J.M. (2010) Fundamentals of Forensic DNA Typing. San Diego: Elsevier Academic Press and other sources.
Glossary of Forensic DNA Terms Definitions are from Butler, J.M. (2010) Fundamentals of Forensic DNA Typing. San Diego: Elsevier Academic Press and other sources.
Guidelines: a set of general principles used to provide direction and parameters for decision making
Heredity: transmission of characteristics from one generation to the next
Heterozygosity: the presence of different alleles at one or more loci on homologous chromosomes; a measure of the diversity of a locus in a population
Heterozygote: an individual having different alleles at a particular genetic locus
Homologous chromosomes: a pair of chromosomes containing the same linear gene sequences, each derived from one parent
Homozygote: an individual having the same (or indistinguishable) alleles at a particular locus due to the inheritance of the same allele from each parent
ISFG: International Society for Forensic Genetics; an organization of scientists from more than 60 countries that meet bi-annually; conference proceedings have been entitled “Advances in Forensic Haemogenetics”, “Progress in Forensic Genetics”, and “Forensic Science International: Genetics Supplement Series”.
Identifiler: a multiplex STR typing kit from Applied Biosystems that co-amplifies 15 STRs and the sex-typing marker amelogenin; the STR loci amplified are D8S1179, D21S11, D7S820, CSF1PO, D3S1358, TH01, D13S317, D16S539, D2S1338, D19S433, VWA, TPOX, D18S51, D5S818, and FGA
Impression evidence Objects or materials that have retained the characteristics of other objects that have been physically pressed against them.
In vitro: Outside a living organism; literally “in glass” meaning biochemical reactions conducted in a test tube or other laboratory apparatus
In vivo: within the cell or organism
Inclusion: a conclusion statement that a biological sample may have originated from a particular source or individual
Inconclusive: data are inadequate to draw any meaningful conclusions; a statement provided as the conclusion when testing results are insufficient or lacking in quality and/or quantity, as defined by the laboratory, for comparison purposes
Indistinguishable mixture: a DNA mixture in which relative peak height ratios/areas as determined from the electropherogram are insufficient to attribute alleles to individual contributor(s); generally observed when a similar amount of DNA from each contributor is present in the original extracted sample such that no clear major or minor contributor can be discerned
Inheritance: the reception of genetic qualities by transmission from parent to offspring or all of the genetic characters or qualities transmitted from parent to offspring
Inhibitor: as related to PCR, any substance that interferes with or prevents the synthesis of DNA during the amplification process
Initial responding officer(s) The first law enforcement officer(s) to arrive at the scene.
Internal size standard (ISS): specific DNA fragments of known length that are used to size other DNA fragments in a sample being measured; in STR testing, an ISS is commonly labeled with a different fluorescent dye not found in the amplification process enabling mixing of the ISS and amplicon without interference in the analysis; sometimes referred to as an internal lane standard (ILS)
Intimate sample: a biological sample from an evidence item that is obtained directly from an individual’s body such as a vaginal swab
Intron: non-coding DNA, which separates neighboring exons in a gene; intron sequences are removed during transcription (the creation of mRNA) so that only exons contribute to gene expression
Investigator(s) in charge The official(s) responsible for the crime scene investigation.
Kinship analysis: comparison of genetic profiles of two or more individuals to evaluate alternative degrees of relatedness; performing DNA evaluations using biological relatives to predict expected genotypes in missing individuals in order to serve as an indirect form of human identification when no direct reference samples are available
Known sample: biological material for which the identity of the donor is established and used for comparison purposes; sometimes referred to as the “K” sample
Latent print A print impression not readily visible, made by contact of the hands or feet with a surface resulting in the transfer of materials from the skin to that surface.
Length polymorphism: variation observed in the length of alleles (e.g., STR loci) between individuals in a population
Like lihood ratio: the probability of the evidence under one proposition divided by the probability of the evidence under an alternative, mutually exclusive proposition; the magnitude of its value expresses the weight of the evidence
Linkage disequilibrium: the non-random association, in a population, of alleles at different loci
Linkage: the non-independent transmission of two genetic units, usually because of proximity on the same chromosome
Loci: plural of locus; pronounced /LOW-sigh/
Locus: a unique physical location of a gene (or a specific sequence of DNA) on a chromosome
Low-copy-number (LCN) DNA testing: the analysis of a small quantity of DNA often conducted by increasing the number of PCR amplification cycles
Low-level or low-template DNA: usually defined as less than approximately 100 picograms (pg) or about 15 human diploid cells
Major contributor: the source of the predominant portion of the DNA in a mixed sample that generates a mixture profile
Marker: a gene or specific DNA sequence of known location on a chromosome; used as a point of reference in the mapping of other loci
Massively parallel sequencing: one of a number of high-throughput DNA sequencing techniques; another name given to next-generation sequencing
Match probability: uses conditional probabilities to address the question “given that a particular DNA profile has been seen in the crime scene evidence and in the suspect, what is the chance of it occurring again?”
Match: genetic profiles show the same types at all loci tested and no unexplainable differences exist
Measurement scale An object showing standard units of length (e.g., ruler) used in photographic documentation of an item of evidence.
Minor contributor: source of the lesser portion of the DNA in a mixed sample that generates a mixture profile
Mitochondrial DNA (mtDNA): a small, circular DNA molecule located in the mitochondria that contains approximately 16,500 nucleotides; the abundance of hundreds of copies of mtDNA in each cell make it useful with samples originating from limited or damaged biological material
Mixed DNA sample: any biological sample containing DNA from more than one individual
Mixture ratio: relative ratio of the DNA contributions of multiple individuals to a mixed DNA typing result as determined by the use of quantitative peak height information; can be used to discern major and minor contributors to a DNA profile
Mixture: DNA typing results originating from two or more individuals
Multiple scenes Two or more physical locations of evidence associated with a crime (e.g., in a crime of personal violence, evidence may be found at the location of the assault and also on the person and clothing of the victim/assailant, the victim's/assailant's vehicle, and locations the victim/assailant frequents and resides).
Multiplex PCR: co-amplification of multiple regions of a genome with more than one set of primers; enables information from the different target sequences to be collected simultaneously
Mutation rate: the relative frequency at which mutations have been observed at a specific genetic locus; generally estimated as the number of mutations observed in parent-offspring pairs divided by the total number of pairs examined
Mutation: any inheritable change in DNA sequence; an alteration or change of an allele at a genetic locus resulting in genetic inconsistency between a biological parent and offspring
NRC II: the second National Research Council report issued in 1996 entitled “The Evaluation of Forensic DNA Evidence”
NRC I: the first National Research Council report issued in 1992 entitled “DNA Technology in Forensic Science”
Nanogram (ng): 10-9 grams (a billionth of a gram); there are approximately 15 human cells in 1 ng of DNA
Negative control: a sample containing only PCR amplification reagents without the addition of template DNA; can be used to detect contamination introduced into the assay during the testing process via reagents, disposables, or handling errors
Next-generation sequencing: non-Sanger-based high-throughput DNA sequencing technologies where millions or billions of DNA strands can be sequenced in parallel; also called massively parallel sequencing
Non-sperm cell fraction: the portion of a sample produced during differential extraction containing DNA from non-sperm cells; also referred to as the epithelial cell or female fraction
Non-template nucleotide addition: the addition of an extra nucleotide not specified by the template to the 3’-end of a PCR product resulting in a fragment that is one base longer than the actual target sequence
Nonporous container Packaging through which liquids or vapors cannot pass (e.g., glass jars or metal cans).
Nuclear DNA: DNA contained within a nucleus of eukaryotic organisms
Nucleotide: a unit of nucleic acid composed of phosphate, ribose or deoxyribose, and a purine or pyrimidine base
Nucleus: the cellular organelle in eukaryotes that contains the genetic material
Null allele: an allele that exists in an individual but is not detected during testing usually due to a mutation in the DNA template that prevents a PCR primer from binding properly; can result in a true heterozygote being called a homozygote
Obligate allele: an allele that must be carried by an individual as determined by relationship analysis of an individual’s ancestry; in a mixed DNA sample, an allele that is foreign to the assumed or known contributor(s)
Off-ladder (OL) allele: STR allele that is of a different length than found in the allelic ladder utilized in typing a particular locus
Off-scale (STR) data: data produced when the emitted fluorescence from the PCR products being measured saturates the detector; may result in flat-topped peaks in the electropherogram for STR alleles and pull-up peaks in one or more color channels corresponding to the off- scale peak; makes data interpretation more complicated
Organic extraction: a method of isolating DNA from cells involving phenol and other organic chemicals
Other responders Individuals who are involved in an aspect of the crime scene, such as perimeter security, traffic control, media management, scene processing, and technical support, as well as prosecutors, medical personnel, medical examiners, coroners, forensic examiners, evidence technicians, and fire and rescue officers.
Partial profile: a DNA profile for which complete typing results are not obtained at all tested loci; may be due to limited DNA template, DNA degradation, inhibition, preferential amplification, and/or stochastic effects
Peak height ratio (PHR): the relative ratio of two peaks at a given locus; may be calculated by dividing the peak height of the longer length allele by the peak height of the shorter length allele or by dividing the shorter peak height by the taller peak; also called heterozygote balance (Hb)
Peak height: the maximum y-axis value obtained for a data peak; for STR analysis using CE and fluorescence detection, it is measured in relative fluorescence units and reflects the quantity of the PCR product being measured
Personal protective equipment (PPE) Articles such as disposable gloves, masks, and eye protection that are utilized to provide a barrier to keep biological or chemical hazards from contacting the skin, eyes, and mucous membranes and to avoid contamination of the crime scene.
Picogram (pg): 10-12 grams (a trillionth of a gram); there are approximately 6 pg of DNA in a single human cell
Polymerase chain reaction (PCR): an in vitro process that yields millions of copies of desired DNA through repeated cycling of a reaction involving the DNA polymerase enzyme
Polymorphism: difference in DNA sequence among individuals. Genetic variations occurring in more than 1% of a population would be considered useful polymorphisms for linkage analysis
Population substructure: the existence of subpopulations with different allele frequencies
Population: a group of individuals residing in a given area at a given time
Porous container Packaging through which liquids or vapors may pass (e.g., paper bags, cloth bags).
Positive control: an analytical control sample that can be used to determine if a test works properly
Post-amplification purification: cleanup of PCR product to reduce or eliminate amplification reaction components from competing with CE electrokinetic injection, which results in higher peaks in the electropherogram
Power of discrimination: the potential power of a genetic marker or set of markers to differentiate between any two people chosen at random; equal to one minus the sum of the square of the genotype frequencies
PowerPlex 16: a multiplex STR typing kit from Promega Corporation that co-amplifies 15 STRs and the sex-typing marker amelogenin; the STR loci amplified are D3S1358, TH01, D21S11, D18S51, Penta E, D5S818, D13S317, D7S820, D16S539, CSF1PO, Penta D, VWA, D8S1179, TPOX, and FGA
PowerPlex Fusion: a multiplex STR typing kit from Promega Corporation that co-amplifies 22 STRs and the sex-typing marker amelogenin
Precision: a measure of the closeness of results when experiments are repeated
Presumptive test A nonconfirmatory test used to screen for the presence of a substance. Projectile trajectory analysis The method for determining the path of a high-speed object through space (e.g., a bullet emanating from a firearm).
Presumptive test: an initial examination of evidence to indicate the possible source of the sample (e.g., blood, saliva, semen, etc.); usually followed up by a confirmatory assay or DNA analysis
Primer: a short preexisting polynucleotide chain, usually 18-30 bases long, which targets a specific region of the template DNA and allows a DNA polymerase to initiate synthesis of a complementary strand
Probabilistic genotyping: use of statistical modeling informed by biological data, statistical theory, computer algorithms and/or probability distributions to infer genotypes and/or calculate likelihood ratios
Probability of exclusion (PE): the percentage of the population that can be excluded as potential contributors to a DNA mixture result
Probability vs. odds: assuming that all outcomes are equally probable, the probability of event A is the number of ways event A can happen divided by the total number of possible outcomes; the odds of an event occurring is the ratio of two competing probabilities—the probability that an event will occur and the probability that it will not occur; if the probability of the event is p, the probability that it will not occur is 1 – p; the odds are therefore p/(1-p)
Product rule: the combination of genotype frequency estimates from multiple loci through multiplying the individual locus genotypes together
Proficiency test: a quality assurance measure used to monitor performance of an analyst and identify areas in which improvement may be needed; can be internal (produced by the agency undergoing the test) or external (produced by an outside test provider); external proficiency tests can be either open or blind
Profile probability: answers the questions “what is the rarity of a specific DNA profile given the alleles observed?” or “what is the chance that a particular profile exists in a population based on allele frequencies?”
Profiler Plus: a commercial STR typing kit from Applied Biosystems that utilizes multiplex PCR to amplify and provide genetic information for 9 STRs and the sex-typing marker amelogenin; the STR loci examined are D3S1358, VWA, FGA, D8S1179, D21S11, D18S51, D5S818, D13S317, and D7S820
Prome ga Corporation: a company based in Madison, Wisconsin that supplies STR typing kits and other reagents to perform DNA analysis
Pull-up: an artifact that may occur during analysis of fluorescently labeled DNA fragments when signal from one dye color channel produces artificial peaks in another, usually adjacent color, at a similar position on the horizontal axis in an electropherogram; sometimes referred to as bleed-through or spectral calibration failure
Q-K comparison: evaluation of question and known sample DNA profiles with one another to reach a decision of inclusion, exclusion, or inconclusive
Qiagen: a German-based company that provides DNA extraction kits and other molecular biology supplies
Quality Assurance Standards (QAS): FBI Director-issued requirements established initially under the DNA Identification Act of 1994 by the DNA Advisory Board; now revised by SWGDAM
Quality assurance (QA): a system of activities whose purpose is to provide to the producer or user of a product or service the assurance that it meets defined standards of quality
Quantitative PCR (qPCR): method used for determining the quantity of DNA template present in a sample through measuring the point at which samples cross a detection threshold relative to a set of standard samples; also known as real-time PCR
RFU: relative fluorescence units
Radiological threat The pending exposure to radiation energy. (This energy can be produced by shortwave x-rays or through unstable isotopes.)
Random Man Not Excluded (RMNE): the probability that a random person would be excluded as a contributor to the observed DNA mixture (DNA profile); answers the question - “how often would a random person unrelated to a true contributor be excluded?”
Random Match Probability (RMP): the probability that an unknown individual in a given population has a particular profile; more appropriately the random match probability is computed conditioned on a known individual observed to have the profile; the unconditional probability is also known as the profile probability
Random Match: a match in the DNA profiles of two sample, where one is drawn at random from the population
Rapid DNA testing: fully automated process of generating a STR DNA profile (“swab in – profile out”) without human intervention; current methods take about 90 minutes and cost about ten times more than conventional DNA testing
Reference sample: a sample (typically blood or buccal swab) taken from a known person that is used for comparison purposes to an evidentiary sample; sometimes referred to as the “K” or known sample
Reproducibility: the ability to obtain the same result when a test or experiment is repeated
STRmix: a probabilistic genotyping software program sold by ESR (Auckland, New Zealand)
SWGDAM: Scientific Working Group on DNA Analysis Methods; formerly known as TWGDAM, Technical Working Group on DNA Analysis Methods; an FBI sponsored group that develops quality assurance standards and guidelines for laboratories in the United States
Sanger sequencing: a method of DNA sequencing named after its inventor Frederick Sanger that involves determining the order of bases in a DNA molecule following selective incorporation of chain- terminating dideoxynucleotides by a DNA polymerase during in vitro DNA replication
Sex chromosomes (X and Y chromosomes): chromosomes that are different in the two sexes and involved in sex determination
Sexual assault kit (SAK): a set of items used by medical personnel to collect and preserve physical sexual assault evidence for use in a criminal investigation
Short tandem repeats (STR): multiple copies of an identical (or similar) DNA sequence arranged in direct succession where the repeat sequence unit is 2 bp to 6 bp in length; because STRs generally occur in the “junk” DNA outside of the constraints of genes, the number of repeat units can vary between individuals in an accordion-like fashion
Single nucleotide polymorphism (SNP): any polymorphic variation at a single nucleotide; most SNPs are biallelic (e.g., either C or T) with the minor allele being observed at least 1% of the time
Single-use equipment Items that will be used only once to collect evidence, such as biological samples, then discarded to minimize contamination (e.g., tweezers, scalpel blades, droppers).
Source attribution: a decision which identifies an individual as the source of the DNA that produced an evidentiary single-source or major contributor profile; the statement if often based on profile frequency estimates that are rarer than some defined value, generally more than the Earth’s population
Spatial calibration: performed when a new capillary array is installed on a CE instrument in order to map the location of fluorescence from each capillary onto the charged-coupled device (CCD) detector
Spectral calibration: an examination of the contribution of overlap in the emission spectrum of fluorescent dyes used for a specific DNA test on a CE instrument; permits the color deconvolution necessary for multi-color STR typing or DNA sequencing; a poor spectral calibration may cause artifact peaks or inaccurate peak height determinations
Sperm cell fraction: the portion of a sample produced during differential extraction containing DNA from sperm cells; generally, from the male perpetrator in the case of sexual assault evidence
Standards: criteria established for quality assurance purposes that place specific requirements on laboratories and analysts; also refers to well- characterized samples that can aid calibration of measurements
Stochastic effects: the observation of intra-locus peak imbalance and/or allele drop-out resulting from random, disproportionate amplification of alleles in low-quantity template samples
Stochastic threshold: the peak height value in a DNA profile above which it is reasonable to assume that, at a given locus, allelic dropout of a sister allele in a heterozygous pair has not occurred in a single-source DNA sample; often used to disqualify loci from consideration with CPI statistical calculations due to the possibility of allele dropout; due to the possibility of shared alleles in mixtures, the presence of allele peaks above the stochastic threshold is no guarantee that allele dropout did not occur in mixed DNA sample profiles
Stutter product: a minor peak primarily appearing one repeat unit smaller than the primary STR allele; results from strand slippage during the amplification process; usually <15% of the height of the true allele
Team members Individuals who are called to the scene to assist in investigation or processing of the scene (e.g., scientific personnel from the crime laboratory or medical examiner's office, other forensic specialists, photographers, mass disaster specialists, experts in the identification of human remains, arson and explosives investigators, clandestine drug laboratory investigators, as well as other experts).
The ta (θ) correction: a method for calculating match probabilities, first described by Balding and Nichols (1994), to allow for population structure in the population for which a frequency database is constructed; it allows match probabilities for subpopulations to be calculated from whole population allele frequencies and avoids the need to assume Hardy-Weinberg equilibrium at the whole population level
Touch DNA: DNA that is transferred to or from surfaces via contact
Trace evidence Physical evidence that results from the transfer of small quantities of materials (e.g., hair, textile fibers, paint chips, glass fragments, gunshot residue particles).
Transient evidence Evidence which by its very nature or the conditions at the scene will lose its evidentiary value if not preserved and protected (e.g., blood in the rain). Unknown/questioned. See comparison samples.
Tri-allelic pattern: an occasional detection of three alleles at a particular STR locus in single-source samples; these copy number variants (CNVs) result from extra chromosome fragments being present in unbalanced (Type 1) or balanced (Type 2) peak heights
TrueAllele: a probabilistic genotyping software program sold by Cybergenetics (Pittsburgh, PA)
Uninterpretable: the inability to interpret or use results for comparisons due to poor or limited data quality, or data that fail to meet quality assurance requirements
Validation: the process of performing and evaluating a set of experiments that establish the efficacy, reliability, and limitations of a method, procedure or modification thereof; establishing recorded documentation that provides a high degree of assurance that a specific process will consistently produce an outcome meeting its predetermined specifications and quality attributes before acceptance for routine use; may include developmental and/or internal validation
Variant allele: a non-standard form of an allele due to a point mutation, an insertion or a deletion relative to other commonly seen alleles
Verification: confirmation by examination that specified requirements have been met
Walk-through An initial assessment conducted by carefully walking through the scene to evaluate the situation, recognize potential evidence, and determine resources required. Also, a final survey conducted to ensure the scene has been effectively and completely processed.
X-STR: short tandem repeat markers found on the X-chromosome that may be useful in addressing some kinship analysis scenarios
X-chromosome: one of the sex chromosomes; normal females possess two copies and males one copy
Y-STR: short tandem repeat markers found on the Y-chromosome that enable male-specific DNA testing; can be useful in cases involving sexual assault and with genetic genealogy to trace male lineages
Y-chromosome: one of the sex chromosomes; normal males possess one copy and females none
Yfiler: a multiplex STR typing kit from Applied Biosystems that co- amplifies 17 segments of the Y-chromosome; the Y-STR loci examined are DYS19, DYS385a/b, DYS389I, DYS389II, DYS390, DYS391, DYS392, DYS393, DYS437, DYS438, DYS439, DYS448, DYS456, DYS458, DYS635, and Y-GATA-H4.
Zygote: cell formed when the nuclear DNA from a father’s sperm cell combines with nuclear DNA in a mother’s egg restoring the diploid chromosome count; a fertilized egg.

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