Lawyers Portal

DNA DICTIONARY

Adventitious match: an association of an evidence DNA profile to the profile of a person who is not the true donor of that profile; can also be referred to as a false inclusion. 

Allele: [al-leel] one of two or more versions of a gene at a particular location (a locus) in the genome. 

Allele drop-in: allelic peak(s) in an electropherogram that are not reproducible across multiple independent amplification events and may be attributable to sporadic contamination events.

Allele drop-out: when a sample is typed, and an allele is not present.  Can occur when an insufficient quantity of DNA is present.  

Allele frequency: the number of times that an allele appears in a data set representative of a particular population.

Allelic ladder: 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.

Amelogenin: a commonly used sex-typing genetic marker targeting the X chromosome and the Y chromosome.  Females (X,X) samples result in a single peak while males (X,Y) have two peaks.

Amplification: an increase in the number of copies of a specific DNA fragment; refers to the use of the PCR technique to produce many more copies of DNA at specific genetic loci.

Analytical threshold: an acceptable “Relative Fluorescence Units” (RFU) level determined to be appropriate for use in the PCR/STR DNA typing process. A minimum threshold for data comparison is identified by the specific forensic laboratory doing the testing through independent 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.

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).

Bayesian approach: defines the probability of an event as the degree of belief in the truth of a proposition that asserts it will happen.

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.

Capillary electrophoresis (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.

Cell: the basic building block of an organism; humans have approximately 30 trillion cells in their body, most containing DNA .

Chromosome: the structure by which hereditary information is physically transmitted from one generation to the next; humans have 23 pairs of chromosomes.

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

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

Conservative: an assignment of the weight of evidence that is believed to favour the defence.

Consensus profile: the resulting DNA profile made up from the list of alleles that are repeated when a sample is amplified multiple times.

Contamination: DNA or other biological material that is introduced to a DNA sample 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.

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): the molecule that carries genetic information for the development and functioning of an organism; 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.

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.

Double helix: the native form of DNA, which looks like a twisted ladder; two linear strands of DNA assume this shape.

Dye blobs: artifact peaks in capillary electropherograms arising from fluorescent dye molecules during the PCR process.

Electropherogram (EPG): 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 size 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.

Epithelial cells: skin cells, vaginal cells, or other cells that are normally found on an inner or outer body surface.

Eukaryote: [yoo-kar-ee-oht] an organism with cells containing a nucleus.

Evidence sample: biological sample collected from a crime scene or people or objects associated with a crime scene; sometimes referred to as a crime scene sample

Exclusion: a conclusion statement that a biological sample did not originate from a particular source or individual.

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.

Extraction:  the process of isolating DNA from the cells of an organism isolated from a sample. It involves breaking open the cells, removing proteins and other contaminants, and purifying the DNA so that it is free of other cellular components.

Fluorescence: the emission of light from a molecule following its excitation by light energy.

Forensic science: the application of scientific knowledge to questions of civil and criminal law, typically through presentation of results from evidence in court.

FST/Theta: the between-person inbreeding coefficient used in subpopulation corrections; commonly equated with the theta (θ) correction

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 locus 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.

Guidelines: a set of general principles used to provide direction and parameters for decision making.

Haplotype:  refers to a set of DNA variants along a single chromosome that tend to be inherited together; applicable to Y Chromosome (YSTR) testing.

Heredity: transmission of characteristics from one generation to the next.

Heterozygote: an individual having different alleles at a particular genetic locus.

Homozygote: an individual having the same (or indistinguishable) alleles at a particular locus due to the inheritance of the same allele from each parent.

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.

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.

Inhibitor: as related to PCR, any substance that interferes with or prevents the synthesis of DNA during the amplification process; an example of an inhibitor is indigo dye in denim.

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, 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.

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.

Kinship analysis: comparison of genetic profiles of two or more individuals to evaluate alternative degrees of relatedness.

Known sample: biological material for which the identity of the donor is established and used for comparison purposes.

Likelihood 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. Typically aligns to the prosecution proposition and defence propositions.

Loci: [low-sigh] plural of locus.

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.

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.

Match: genetic profiles show the same types at all loci tested and no unexplainable differences exist.

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?”

Minor contributor: source of the lesser portion of the DNA in a mixed sample that generates a mixture profile.

Mitochondrial DNA (mtDNA): a DNA molecule located in the mitochondria of the cell; the abundance of hundreds of copies of mtDNA in each cell make it useful with samples originating from limited or damaged biological material; requires a specialised form of DNA testing.

Mixed DNA sample: any biological sample containing DNA from more than one individual.

Mixture: DNA typing results originating from two or more individuals.

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.

Multiplex PCR: co-amplification of multiple regions of a genome.

Mutation: an alteration or change of an allele at a genetic locus resulting in genetic inconsistency between a biological parent and offspring.

Nanogram (ng): a billionth of a gram.

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.

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.

Nuclear DNA: DNA contained within a nucleus of a cell.

Nucleus: the cellular organelle in eukaryotes that contains the genetic material.

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: the maximum y-axis value obtained for a data peak; measured in relative fluorescence units (RFU) and reflects the quantity of the PCR product being measured.

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).

Phenotype:  the set of observable characteristics or traits of an organism, includes features such as eye colour and hair colour.

Picogram (pg): 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.

Population: a group of individuals residing in a given area at a given time.

Population substructure: the existence of subpopulations with different allele frequencies.

Positive control: an analytical control sample that can be used to determine if a test works properly.

Power of discrimination: the potential power of a genetic marker or set of markers to differentiate between any two people chosen at random.

PowerPlex 21: a multiplex STR typing kit from Promega Corporation that co-amplifies 15 STRs and the sex-typing marker amelogenin.

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.

Probabilistic genotyping: use of statistical modelling informed by biological data, statistical theory, computer algorithms and/or probability distributions to infer genotypes and/or calculate likelihood ratios.

Probability: 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.

Odds: 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).

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).

Pull-up: an artifact that may occur during analysis of fluorescently labelled DNA fragments when signal from one dye colour channel produces artificial peaks in another; sometimes referred to as bleed-through.

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.

Random Match Probability (RMP): the probability that an unknown individual in a given population has a particular profile.

Reference sample: a sample (typically blood or buccal swab) taken from a known person that is used for comparison purposes to an evidentiary sample.

Sex chromosomes (X and Y chromosomes): chromosomes that are different in the two sexes and involved in sex determination.

Sexual assault investigation 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 DNA sequence arranged in direct succession, the number of repeat units can vary between individuals.

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.

Sperm cell fraction: the portion of a sample produced during differential extraction containing DNA from sperm cell.

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.

STRmix: a probabilistic genotyping software program sold by ESR (Auckland, New Zealand).

Stutter product: a minor peak primarily appearing one repeat unit smaller than the primary STR allele; results from strand slippage during the amplification process.