Glossary of Terms Frequently Encountered in Forensic DNA

Allele: A different form of a gene at a particular locus. For example, one copy of a specific short tandem repeat (STR) region might have 10 repeats, while the other copy might have 11 repeats. These would represent two alleles of that STR region.
Allele Frequencies: Term used to characterize genetic variation of a species population.
Allelic dropout: Failure to detect an allele within a sample or failure to amplify an allele during PCR.
Amelogenin: A gene present on the X and Y sex chromosomes that is used in DNA identification testing to determine the gender of the donor of the DNA in a biological sample.
Amplification: Producing multiple copies of a chosen DNA region, usually by PCR (Polymerase Chain Reaction).
Autosomal: Chromosomes which are not sex chromosomes.
Base Pairing: A,T,C and G are molecular building blocks of DNA that only continue in specific "base" pairs, e.g.,A only pairs with T, and C only pairs with G
Bases: The four building blocks of DNA are called bases. The building blocks are Cytosine, Guanine, Thymine, Adenine and are commonly referred to as C, G, T, A.
Biological Evidence: Evidence commonly recovered from crime scenes in the form of hair, tissue, bones, teeth, blood or other bodily fluids.
Cambridge Reference Sequence (CRS): A "master template" of the HV regions of mitochondrial DNA.
Capillary Electrophoresis (CE): The platform for CE uses narrow silica capillaries (or tubes) containing a polymer solution through which the negatively charged DNA molecules migrate under the influence of a high voltage electric field.
Cell: The smallest component of life capable of independent reproduction and from which DNA is isolated for forensic analysis.
Chain of Custody: A record of individuals who have had physical possession of the evidence and the process used to maintain and document the chronological history of the evidence.
Chromosome: Located in the cell nucleus, the biological structure by which hereditary information is physically transmitted from one generation to the next.
CODIS: Combined DNA Index System. A collection of databases of DNA profiles obtained from evidence samples from unsolved crimes and from known individuals convicted of particular crimes.
CODIS core loci: Thirteen STR (short tandem repeat) sequences that have been selected for the Combined DNA Index System (CODIS).
Contamination: The undesirable transfer of material to physical evidence (DNA) from another source.
Controls: Tests designed to demonstrate that a procedure worked correctly and performed in parallel with experimental samples.
Degradation: The fragmenting, or breakdown, of DNA by chemical or physical means.
Differential extraction: A procedure in which sperm cells are separated, or extracted, from all other cells in a sample.
DNA (Deoxyribonucleic acid): Often referred to as the "blueprint of life," DNA is the genetic material present in the nucleus of cells which is inherited half from each biological parent. An individual's DNA is unique except in cases of identical twins.
DNA Analysis: The process of testing to identify DNA patterns or types. In the forensic setting, this testing is used to exclude or include individuals as possible sources of body fluid stains (blood, saliva, semen) and other biological evidence (bones, teeth, hair). This testing can also be used to indicate parentage.
DNA mixtures: A sample that contains the DNA of more than one individual.
DNA Profile: The result of determining the relative positions of DNA sequences at several locations on the molecule. Each person (except identical twins) has a unique DNA profile when used in the context of the CODIS database, which evaluates 13 specific DNA locations.
Double Helix: The shape the DNA assumes after it replicates during cell life.
Electropherogram: The graphic representation of the separation of molecules by electrophoresis or other means of separation.
Electrophoresis: A method of separating large molecules (such as DNA fragments) from a mixture of similar molecules. An electric current is passed through a medium at a different rate, depending on its electrical charge and size. Separation of DNA markers is based on these differences.
Elimination: eference samples - A term used to describe a sample of known source taken for comparison purposes.
Excluded: Two samples cannot have come from the same source.
Exclusion: A DNA test result indicating that an individual is excluded as the source of the DNA evidence.
Forensic Science: The application of science to analyze evidence involved in criminal and civil litigation.
Fractions: The result of the differential extraction; separating sperm cells from all other DNA material.
Gene: The basic unit of heredity; a functional sequence of DNA in a chromosome.
Genetic loci: Places in the genetic material of an organism where specific DNA sequences can be found.
Genetics: The study of the patterns of inheritance of specific traits.
Genome: All the genetic material in the chromosomes of a particular organism; its size is generally given as its total number of base pairs.
Genotype: The genetic constitution of an organism, as distinguished from its physical appearance (its phenotype). The designation of two alleles at a particular locus is a genotype.
Haplotype: A way of denoting the collective genotype of a number of closely linked loci on a chromosome.
Hardy-Weinberg (Equilibrium): In a large random intrabreeding population, not subjected to excessive selection or mutation, the gene and genotype frequencies will remain constant overt time. The sum of p²+2pq+q² applies at equilibrium for a single allele pair where p is the frequency of the allele A, q is the frequency of a, p² is the frequency of genotype AA, q² is the frequency of aa, and 2pq is the frequency of Aa.
Heredity: The transmission of characteristics from one generation to the next.
Heteroplasmy: The presence of more than one mtDNA type within a single individual.
Heterozygous: If two alleles are different at one locus, the person is heterozygous at that genetic location.
Homozygous: If two alleles at a locus that are indistinguishable, the person is homozygous at that genetic location.
Hypervariable: An area on the DNA which can have many different alleles in differing sequences.
Included: Two samples could have come from the same source.
Inclusion (Failure to Exclude): The inability to exclude an individual as a possible source of a biological sample.
Inconclusive: A situation in which no conclusion can be reached regarding testing done due to one of many possible reasons (e.g., no results obtained, uninterpretable results obtained, no exemplar/standard available for testing).
Intimate Sample: An intimate sample is generally referred to a biological sample obtained from a source other than the mouth (saliva) and head (hair).
John Doe Warrant: A warrant used when crime scene evidence yields a DNA profile but the individual corresponding to the DNA profile is unknown. In lieu of the suspect's name, the warrant will be filed as "John Doe" and cite the DNA profile.
Junk DNA: Stretches of DNA that do not code for genes; "most of the genome consists of junk DNA."
Known samples: A DNA sample for which the source is known. These samples are generally obtained from the victim and/or suspected perpetrator of a crime, as well as from other persons whose DNA might be reflected when samples of the evidence are analyzed (could include a boyfriend, husband, or other third-party). These samples are also referred to as reference samples, since they serve as a reference to which the unknown DNA samples are compared with the goal of identifying the source of the unknown DNA samples.
Length heteroplasmy: The presence of mtDNA molecules that differ in length.
Likelihood ratio: The ratio of two probabilities of the same event under different hypotheses. In DNA testing often expressed as the ratio between the likelihood that a given profile came from a particular individual and the likelihood that it came from a random unrelated person. Note that in this case the likelihood of each event does not add to give 1 (100% likelihood) as it does not incorporate the possibility of error or that the profiles came from twins or other near relatives.
Linkage Equilibrium: When two or more genetic loci appear to segregate randomly in a given population. The genotypes appear randomly with respect to each other.
Locus (pl. loci): The specific physical location of a gene on a chromosome.
Low copy number: Refers to examination of less than 125 pg (picograms) of input DNA.
Major contributor profile: A DNA profile where multiple individuals have contributed biologic material and one individual’s DNA profile is more apparent.
Match: Genetic profiles are said to "match" when they have the same allele designations at every loci.
Mitochondrial DNA (mtDNA): The DNA found in the many mitochondria found in each cell of a body. The sequencing of mitochondrial DNA can link individuals descended from a common female ancestor.
mtDNA types: A mtDNA type is the sequence of a region of mtDNA. Common sources of mtDNA are hairs, skeletal remains, and teeth.
Multiplexed: A system for analyzing several loci at once.
No results: A situation in which no interpretable results are obtained from testing a DNA sample. A finding of no results can be due to the absence of DNA, insufficient DNA, or substances that inhibit the PCR process, among others.
Nuclear DNA: The DNA found in the nucleus of a cell.
Nucleus: The cellular organelle that contains most of the genetic material.
Partial profile: DNA evidence that does not yield identifiable results in all 13 core loci.
Paternal inheritance: Genetic material which is inherited from one’s father.
PCR Inhibitors: A substance that interferes with the Polymerase Chain Reaction (PCR) process.
Polymerase Chain Reaction (PCR): A process used in DNA identification testing in which one or more specific small regions of the DNA are copied using a DNA polymerase enzyme so that a sufficient amount of DNA is generated for analysis.
Polymorphic: Variable, more than one kind.
Polymorphism: Variations in DNA sequences in a population that are detected in human DNA identification testing.
PopStats: FBI CODIS software program used to perform statistical DNA match estimates.
Probability: The chance of observing a particular future event; a simple ratio of the number of observed events divided by the total number of possible events.
Probability calculations: Predictions based on small sampling of a larger population.
Probability of Exclusion: The probability that a random individual would be excluded as the source of analyzed DNA evidence.
Probability of inclusion: The probability that a random individual would be included as a potential source of analyzed DNA evidence.
Product rule: The product rule calculates the expected chance of finding a given STR (short tandem repeat) profile within a population by multiplying the frequency of occurrence of the combination of alleles (genotype) found at a single locus, by the frequency of occurrence of the genotype found at the second locus, by the frequency of occurrence, in turn, of each of the other genotypes at the remaining STR loci.
Proficiency testing: A DNA proficiency test uses biological samples to assess a lab analyst's ongoing competency and the laboratory's ability to produce accurate results.
Pull-up: Specifically related to analysis software, a peak seen in one color that is not due to the presence of DNA, but to incorrect compensation for the spectral overlap of the four dyes used in detecting multiple loci in one reaction.
Quality Assurance (QA): A program conducted by a laboratory to ensure accuracy and reliability of tests performed.
Quality Assurance Standards: The standards originated from the DNA Advisory Board and are maintained by the FBI; they place specific requirements on labs involved in forensic DNA analysis, both casework and convicted offender databasing.
Random match probability: The probability that the DNA in a random sample from the population has the same profile as the DNA in the evidence sample.
Sequence (or site) heteroplasmy: Presence of mtDNA molecules that have different nucleotides at the same address.
Sequencing: Determination of the order of base sequences in a DNA molecule.
Short Tandem Repeats (STR): Multiple copies of a short identical DNA sequence arranged in direct succession in particular regions of chromosomes.
Single source profile: A DNA profile where only one individual has contributed biologic material.
Standard Operation Procedures (SOP): A prescribed procedure to be followed routinely.
Stochastic effects: Being or having a random variable.
Short Tandem Repeat (STR) typing: DNA analysis method which targets regions on the chromosome which contain multiple copies of an identical DNA sequence in succession.
Stutter: A minor band or peak appearing one repeat unit smaller than a primary STR (short tandem repeat) allele. Occasionally, the repeat unit is larger than the primary allele.
Substrate: Any background material upon which biological sample has been deposited (e.g., clothing, glass, wood, upholstery).
SWGDAM: Scientific Working Group of DNA Analysis and Methods, formerly called TWGDAM (Technical Working Group on DNA Analysis and Methods).
Threshold value: A relative fluorescent unit (RFU) value that must be exceeded to make an allele call. This value will vary among laboratories.
Uninterpretable: Results which might be reported by the laboratory when alleles can not be interpreted.
Upper Bound Frequency Estimates: an estimate of the percentage of individuals who could be potential contributors of a mtDNA or Y STR profile.
Validation: The process of extensive and rigorous evaluation of DNA methods before acceptance for routine use.
Variant: A dissimilarity in the commonly occuring sequence of a gene.
Y-STR: short tandem repeats located on the Y chromosome