Glossary of Forensic DNA Terms
ABI Genetic Analyzer
This is a highly sophisticated capillary electrophoresis instrument that analyzes STR fragments of DNA according to their length and generates data used for forensic DNA identification.
An allele is one of many forms that a genetic marker at a particular locus may take. DNA markers may differ in size or in the arrangement of the molecules (A, T, C and G), resulting in two or more alleles. These differences make each person’s DNA profile unique. The data typically shows two allele sizes (shown as numbers) at each locus for each person, one allele contributed by the person’s biological mother and the other allele contributed by the biological father. By convention, when the two alleles at a locus are the same, the number is shown only once.
American Society of Crime Laboratory Directors/Laboratory Accreditation Board
This is an organization whose assessors inspect forensic labs for accreditation.
Capillary Electrophoresis is a method in which DNA fragments are separated according to their sizes as they travel through a narrow polymer-filled tube.
Chain of Custody
Chain of Custody refers to the record of the documentation process that assures the samples are clearly accounted for by all who handled the evidentiary item(s). The chain of custody allows test results to be legally defensible.
Chromosomes are bundles of DNA molecules that are found in a cell and are held together by proteins. Humans have 46 chromosomes that are arranged in pairs, each pair consisting of one chromosome inherited from the mother and one chromosome inherited from the father. There are 22 pairs of non-sex chromosomes and 1 pair of sex chromosomes (XX or XY).
CODIS: Combined DNA Index System
CODIS is a national database of DNA profiles of felons and crime scene evidence.
Deoxyribonucleic acid (DNA) is a chemical substance found in cells of all living things. DNA, which is inherited from the biological parents, is the basis of heredity. It contains the code for all our physical attributes as well as the instructions for all functions of the body, including growth, development, and replication. DNA is composed of four different chemicals (A, T, C, and G) that function like letters in the code.
A DNA profile is a record of a person’s combination of DNA markers. It serves as a useful tool for genetic identification.
DNA polymerase is an enzyme used to catalyze the synthesis of double-helix strands of DNA for PCR duplication.
An electropherogram is the graphical representation of the raw data generated by the genetic analyzer. The displayed peaks indicate both size (measured in base pairs) and intensity (measured in relative fluorescent units or RFUs) of the DNA fragments.
An exclusion result means that the DNA test has revealed a non-match in the DNA profiles of the tested unknown evidentiary sample compared to the known sample from an individual.
A gene is a single unit of information in DNA. The four molecules that comprise DNA (A, T, C, and G) are found in specific arrangements that spell out instructions for the cell, much like letters form words and sentences. The cells of the body interpret instructions from the genes in order to perform life functions and produce physical characteristics, such as eye color and blood type.
An inclusion result means that the DNA test has revealed a match in the DNA profiles of the tested unknown evidentiary sample compared to the known sample from an individual.
Locus (plural loci)
A locus indicates the position of a gene on a chromosome. We use loci to describe the genetic markers we use for DNA testing: for example, D3S1358 the D marker is on the 1358th locus described on chromosome 3. The letters in the marker name provide other information about the DNA molecule where the marker is found.
Mitochondrial DNA (mtDNA)
Mitochondrial DNA is found outside the nucleus in a cell and is maternally inherited and therefore not unique to an individual. Because there are thousands of copies of mtDNA per cell and it is less prone to degradation than nuclear DNA, mtDNA is often used to obtain profiles from bone, teeth, hair without roots, and other difficult samples.
A mixture profile means that there are more than two alleles present at a given locus, indicating there is more than one contributor to the sample. Mixture profiles are common in forensic cases and must be interpreted carefully.
Nuclear DNA is DNA that is found inside the nucleus of cells, as opposed to mitochondrial DNA [see mitochondrial DNA entry]. Nuclear DNA stores the information necessary for passing genetic attributes to from parents to offspring. Every cell of the human body contains the same nuclear DNA, except for red blood cells (which lack nuclei). The 13 CODIS loci used for human identification are found in nuclear DNA, which is the most common form of DNA used in forensic testing
Polymerase Chain Reaction is a process of repeated cycles of heating and cooling in order to separate strands of DNA for duplication, yielding millions of copies of target DNA sequences for analysis.
‘Relative Fluorescent Unit” is a measure of the intensity/quantity of the DNA fragment as detected by the gene fragment analyzer.
Serology is the study and analysis of body fluids. In forensic cases, serology is one of the first steps in determining the origin of a stain, such as blood, semen or saliva.
Short Tandem Repeats (STR)
STR is a type of DNA marker. This short sequence of DNA is repeated multiple times in succession. The number of times STRs are repeated distinguishes a person from others in the population. This is what makes STRs useful for human identification purposes.
Y-STR is DNA found on the Y-chromosome that is paternally inherited and therefore not unique to an individual. Y-STR testing is often used to help decipher a male DNA profile in a mixture of male and female in a DNA sample.