ABO blood typing - A commonly used genetic typing test that uses antibodies to detect variations on the surface of human red blood cells. Individuals are typed as having A, B, O, or AB type blood by testing liquid or stains from body fluids (e.g., blood, saliva, vaginal secretions). One out of every three randomly selected pairs of people have the same ABO blood type.
Allele - A different form of a gene at a particular locus. The characteristics of a single copy of a specific gene, or of a single copy of a specific location on a chromosome. For example, one copy of a specific STR region might have 10 repeats, while the other copy might have 11 repeats. These would represent two alleles of that STR region.
Allelic dropout - Failure to detect an allele within a sample or failure to amplify an allele during PCR.
Alternate light source (ALS) - Equipment used to produce visible and invisible light at various wavelengths to enhance or visualize items of evidence (fluids, fingerprints, clothing fibers, etc.). The light will cause possible biological stains to change color or fluoresce, assisting in the location process.
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.
Biallelic - Pertaining to both alleles, e.g. single nucleotide polymorphisms display two alternate forms and are biallelic.
Blind testing - In a blind test, analysts do not know they are being tested. In most forensic DNA laboratories, blind tests are not used.
In practice, it is almost impossible to design and implement an effective blind proficiency testing program in forensic science. Most attempts have failed because they could not produce an effective case scenario with realistic representation of the pre-laboratory steps. Others failed because the analyst recognized that the supposed "evidence" was a manufactured artifact. Overall, it has proven impossible to realize the theoretical extra benefits of blind testing, and resources have been devoted to promoting better quality external open tests.
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. Important advantages of the CE technique, compared to slab gel electrophoresis, include quicker and more easily automated analyses.
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. (Documents can include, but are not limited to, name or initials of the individual collecting the evidence, each person or entity subsequently having physical possession of it, dates the items were collected or transferred, where the item(s) were collected from, agency and case number, victim's or suspect's name (if known), and a brief description of the item.
Chromosome - A physical structure in the cell nucleus. It consists of a tightly coiled thread of DNA with associated proteins and RNA. The genes are arranged in linear order along the DNA.
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. Contributions to this database are made through State crime laboratories and the data are maintained by the FBI.
CODIS core loci - Thirteen STR (short tandem repeat) sequences that have been selected for the Combined DNA Index System (CODIS).
Cold hit - When CODIS recognizes a match between an offender and forensic profile, it is referred to as a "cold hit".
Competency - The combination of demonstrated knowledge, skills and abilities.
Contamination - The unwanted transfer of material from another source to a piece of physical evidence.
Cytoplasm - The viscid, semifluid matter contained within the plasma membrane of a cell, excluding the nucleus.
Degradation - The fragmenting, or breakdown of DNA by chemical or physical means.
DNA (deoxyribonucleic acid) - Often referred to as the "blueprint of life," DNA is the genetic material present in the nucleus of cells that is inherited half from each biological parent.
Dideoxy sequencing - Dideoxynucleotide sequencing, also known as the 'Sanger method,' is a technique that uses dideoxyribose instead of deoxyribose to stop the synthesis of a complementary DNA strand at various points when sequencing.
Differential extraction - A procedure in which sperm cells are separated, or extracted, from all other cells in a sample.
DNA fingerprinting - Analyses of the lengths of the fragments reveal that when looking at multiple VNTRs within and between individuals, no two people have the same assortment of lengths. This technique became known to the public as "DNA fingerprinting" because of its powerful ability to discriminate between unrelated individuals.
DNA mixtures - A sample that contains the DNA of more than one individual.
DNA profile - For an individual, the DNA types present at a particular set of tested DNA regions.
Electropherogram - A representation of alleles in the form of peaks after separation by electrophoresis and electronic detection.
Electrophorese - To separate molecules on the basis of electric charge, size, or other physical properties.
Elimination sample - An elimination sample is one of known source taken from a person who had lawful access to the scene (e.g., fingerprints from occupants, tire tread impressions from police vehicles, footwear impressions from emergency medical personnel) to be used for comparison with evidence of the same type.
Evidentiary 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.
Exclusion - The elimination of an individual as the source of a biological sample. This occurs when one or more types from a specific location in the DNA of a known individual are not present in the type(s) for that specific location in the DNA obtained from an evidence sample.
Exogenous DNA - DNA originating outside an organism that has been introduced into the organism.
External testing - An external test is one that is created and administered by an outside agency.
Haplotype - A way of denoting the collective genotype of a number of closely linked loci on a chromosome.
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.
HIPAA - Health Insurance Portability and Accountability Act of 1996.
HLA DQ-alpha - A polymorphic gene in the Human Leukocyte Antigen (HLA) region of chromosome 6 that has been well studied and analyzed for many purposes, including paternity testing, transplantation biology, and human DNA identification testing.
Homozygous - If two alleles at a locus that are indistinguishable, the person is homozygous at that genetic location.
Hybridization - The process of joining two complementary strands of DNA to form a double-stranded molecule.
Hypervariable - An area on the DNA which can have many different alleles in differing sequences.
Inclusion (failure to exclude) - The inability to exclude an individual as a possible source of a biological sample. This occurs when all types from a specific location in the DNA of a known individual are also present in the types for that specific location in the DNA obtained from an evidence 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).
Internal testing - An internal test is one that is created and administered by the laboratory itself.
Intimate sample - An intimate sample generally refers to a biological sample obtained from a source other than the mouth (saliva) and head (hair).
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.
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.
National DNA Index System (NDIS) - Authorized by the DNA Identification Act of 1994, the FBI administers this national index. NDIS compares DNA profiles associated with a crime scene to DNA profiles collected from known convicted offenders, as well as to other crime scene profiles. When the DNA profiles are uploaded to NDIS, they are searched against the other DNA profiles submitted by other participating states.
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 other reasons.
Non-conformances - Inconsistencies in laboratory practices that do not meet accreditation standards.
Nonmatch - An individual is eliminated as the source of a biological sample. This occurs when one or more types from a specific location in the DNA of a known individual are not present in the type(s) for that specific location in the DNA obtained from an evidence sample.
Objective test - A test which having been documented and validated is under control so that it can be demonstrated that all appropriately trained staff will obtain the same results within defined limits. These defined limits relate to expressions of degrees of probability as well as numerical values.
Offender Index - DNA profiles developed from qualifying offenders and uploaded into CODIS are maintained in the offender index of the database.
Partial profile - DNA evidence that does not yield identifiable results in all 13 core loci.
Partially degraded DNA - Forensic DNA evidence exposed to environmental conditions that may prevent it from yielding a usable profile.
PCR inhibitors - A substance that interferes with the Polymerase Chain Reaction (PCR) process. Examples of PCR inhibitors include dyes, soil, chemicals, and heme (hemoglobin).
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.
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 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.
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.
Reference sample - A standard/reference sample is material of a verifiable/documented source which, when compared with evidence of an unknown source, shows an association or linkage between an offender, crime scene, and/or victim (e.g., a carpet cutting taken from a location suspected as the point of transfer for comparison with the fibers recovered from the suspect's shoes, a sample of paint removed from a suspect vehicle to be compared with paint found on a victim's vehicle following an accident, or a sample of the suspect's and/or victim's blood submitted for comparison with a bloodstained shirt recovered as evidence).
SDIS - State DNA Index System containing the state-level DNA records uploaded from local laboratory sites within the state. SDIS is the state's repository of DNA identification records and is under the control of state authorities. The SDIS laboratory serves as the central point of contact for access to NDIS. The DNA Analysis Unit I (DNAUI) serves as the SDIS laboratory for the FBI.
Uninterpretable - Results that might be reported by the laboratory when alleles can not be interpreted.
UV light source - Use of an ultraviolet light source to enhance or visualize potential items of evidence (fluids, fingerprints, clothing fibers, etc.). The light will cause possible biological stains to change color or fluoresce, assisting in the location process.