Acid Phosphatase
An enzyme marker used as a presumptive test for semen. Acronym is AP
Allele
One of a series of alternative forms of a gene (or STR) at a specific locus in a genome.
Allelic Ladder
A mixture of the most common STR alleles that are used to compare with the amplified samples.
Amelogenin
A PCR amplified locus that varies between males (212 and 218 bp) and females (212 bp only)
Amplification
An increase in the number of copies of a specific DNA fragment; see PCR
Amylase
A presumptive test for saliva; also may be present in high concentrations in feces
Autosomal
A DNA profile from 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 pairs (bp)
In DNA the bases adenosine (A) always pairs with Thymine (T) and Cytosine (C) always pairs with Guanine (G). Because DNA is a double stranded helix, the size of DNA molecules is often expressed in base pairs.
Buccal Cells
Cells derived from the inner cheek lining. These cells can be gently scraped from the inner cheek surface with a swab and are present in the saliva.
Buffer
A solution that resists changes in pH (acidity/alkalinity)
Chromosome
Structures, composed of DNA and protein, found in the nucleus of the cell. Most human cells contain 46 chromosomes; 23 derived from the mother and 23 derived from the father. Gametes (sperm or egg cells) contain 23 chromosomes until conception.
Crime specimen
A biological specimen (blood, semen, tissue, hair, etc.) left at the crime scene or, often the blood of a victim.
Denaturation
Use of heat or alkali to disrupt hydrogen bonds that hold the two strands of the DNA double helix together.
Diploid
Two complete genomes in every cell, one derived from mom and the other from dad.
DNA sequence
Order of bases in a DNA molecule.
DNA
Deoxyribonucleic acid. DNA is the double helix molecule of hereditary. It is composed of 4 bases or building blocks: adenine (A), thymine (T), cytosine (C) and guanine (G). These bases are strung along two opposing strands, like pearls on a string. The A is always bound to T and G is always bound to C by week hydrogen bonds. The specific order of bases is the DNA sequence.
Electropherogram
The pattern of fluorescent peaks produced by alleles during electrophoresis. When printed, these appear as peaks on a graph. Collectively, this pattern is the DNA profile.
Electrophoresis
The process of separating charged molecules, such as fragments of DNA, in an electric field. The electric current is passed through an agarose gel, which allows the DNA fragments to separate, by size.
Enzyme
A protein that can speed up a specific chemical reaction without being changed or consumed in the process. Examples of forensically important enzymes are acid phosphatase and amylase.
Exclusion
When the DNA from a crimes scene fails to match that of a suspect. Inclusions are probability statements, exclusions are absolute.
Fluorescence
Molecules that are fluorescent emit light upon excitation with light. Radiator antifreeze contains a fluorescent dye that causes it to appear to glow yellow-green. In practice, DNA molecules are tagged with fluorescent molecules. Upon excitation with laser light, fluorescent genetic analyzers can detect and size the tagged DNA. Since it is possible to tag DNA with different fluorescent molecules that emit light in different regions of the spectrum, it is possible for instruments to detect and distinguish between DNA fragments of similar size.
Gene
Unit of heredity, region of DNA that encodes or regulates the expression of a protein.
Genome
All of the genetic material or DNA in a cell.
Genotype
The genetic makeup of an organism
Heterozygous
An individual that containing two different allele (genetic traits), one from mom and the other from dad, for a given trait. In RFLP this means that an individual produces two different bands on a gel.
Homozygous
An individual that contains two of the same alleles (genetic traits), one from mom and the other from dad, for a given trait. In RFLP this means that an individual produces a single band on a gel.
Inclusion
A probability statement that describes a DNA match between a crime specimen and suspect.
Kilobase (kb)
Unit of length for DNA fragments equal to 1000 base pairs
Ladder
A mixture of the most common STR alleles that are used to compare with the amplified samples.
Linkage
A measure of association between two loci. Loci on different chromosomes are non-linked. Those that are close together on the same chromosome are closely linked and are likely to be inherited together.
Locus (pl., loci)
The location of a genetic trait on a chromosome.
Multiplexing
A sequencing approach that uses several pooled samples simultaneously, greatly increasing sequencing speed.
Mutation
Any heritable change in DNA sequence.
Nanogram (ng)
One billionth of a gram
Nucleotide
A building block of DNA or RNA
Off-Ladder Alleles
An allele that is not included in the allelic ladder. These can be true, rare alleles or artifacts.
p30 Antigen
A confirmatory test for human semen; also known as PSA or prostatic specific antigen
PCR
Polymerase Chain Reaction. A method of amplifying or copying a defined region or target of DNA by repeated cycles of denaturation (heat), annealing and extension. After 30 cycles, a single target DNA can be copied over a billion fold.
Polymerase (DNA)
Enzymes that catalyze the synthesis of DNA on preexisting nucleic acid templates. In forensic DNA we are talking about a heat tolerant enzyme known as Taq polymerase.
Polymorphic
A highly variable genetic trait in a population. An RFLP locus that is polymorphic will have many possible size variants between individuals.
Population
A group of interbreeding individuals. In forensic DNA, we tend to use race as a proxy for population.
Primer
A short length of synthetic DNA used to initiate PCR.
Profile
A description of an individual or evidence resulting from the examination of one or more polymorphic loci
Protein
A large molecule composed of one or more chains of amino acids in a specific order; the order is determined by the base sequence of nucleotides in the gene coding for the protein. Proteins are required for the structure, function, and regulation of the body cells, tissues, organs, and each protein has unique functions.
Pull-up
An artifact resulting from an intense peak in one color channel, which is detected as a minor peak in another color channel.
Race (in genetics)
A population or group that has a characteristic set of gene frequencies
RFLP
Restriction Fragment Length Polymorphism. This method is no longer in use but may appear in older cases. In this method, the target DNA is cut, using a restriction enzyme resulting in a range of fragment sizes. The DNA fragments are separated by agarose electrophoresis and transferred (blotted) to a nylon membrane. The nylon bound DNA is hybridized with a labeled (radioactive or enzyme) DNA probe. A DNA profile is produced by exposure to X-ray film.
rfu
Relative fluorescence units. The scale, usually from 0-6,000, used to measure the relative intensity (amounts) of a signal. Because the intensity of a signal can vary depending on the intensity of the excitation, the amount of light absorbed, re-absorption, quenching and other biophysical parameters, and the measured intensity is relative.
rfu threshold
A minimum value, usually 150 rfu, programmed into Genotyper, as a minimum to call a peak an allele.
Semen
Seminal fluid containing sperm (or not in the case of vasectomy) and other secretions of the male urogenital tract
Spike
A peak on an electropherogram that can be caused by dust or an electrical disturbance, resulting in a peak that is seen across all color channels.
STR
Short Tandem Repeat; short, repetitive sequence elements of 2-5 bases. STR's used in DNA profiling are polymorphic, which is to say that the number of repeat elements varies between individuals in a population. STR alleles can be sized by polyacrylamide gel electrophoresis (PAGE) or capillary electrophoresis (CE).
Stutter
An artifact of STR amplification, in which the n-1 allele (n is the true allele; n-1 is the true allele minus one repeat unit) produces a signal that is 5-10% of the peak height of the true allele. For some loci, this can be as high as 20%. Stutter peaks can rarely occur at the n-2 and very rarely at the n-3 position.
Tandem repeat
The end-to-end duplication of short, identical DNA sequences in the genome.
Taq polymerase
An enzyme isolated from bacteria that live at high temperature that catalyzes the polymerase chain reaction.
Template
Genomic DNA, from crime scene samples or reference samples that are the targets for PCR amplification.
Variant Allele
An STR allele with an incomplete allele. A common THO1 variant allele is 9.3. This allele has 9 full repeats plus 3 nucleotides.
Y-STR
Short Tandem Repeat (STR) profile from the Y-chromosome. Because only men carry the Y-chromosome, these profiles are specific for men. Because the Y-chromosome is passed from father to son, usually without change, all males in the paternal line of decent would probably have an identical Y-STR profile.