Common DNA Forensics Terminology
Allele: A unique form of a single gene that occupies the same locus in the human genome. For
example, one allele of the hair-color gene will produce brown hair and another allele will produce
blonde hair.
Allele Frequency: The frequency with which one form of a gene (an allele) occurs within a given
population.
Polymorphism: Multiple alleles of a gene within a population, usually expressing different
variations (phenotypes). Some polymorphisms, in hypervariable regions of the human genome, are
particularly useful in identifying individuals.
Nucleotides: The rungs of the DNA ladder contain four nucleotides that repeat. The sequence of
adenine (A), thymine (T), guanine (G) and cytosine (C) carries genetic information.
Base Pairs: Two nucleotides on opposite, complementary DNA strands that are connected by
hydrogen bonds. Adenine (A) forms a base pair with thymine (T) and guanine (G) with cytosine (C).
Locus: The position of a gene or chromosome segment on a chromosome. Alleles are located at
identical loci on homologous chromosomes. Humans have 22 pairs of homologous non-sex
chromosomes, called autosomes.
Polymerase Chain Reaction (PCR): A scientific technique used to amplify (or replicate) a
single or few copies of a piece of DNA recovered from a crime scene. The process creates
thousands to millions of copies of a particular DNA sequence for analysis.
Short Tandem Repeats (STR): Occur in DNA when a pattern of two or more nucleotides
(molecules that make up DNA’s structure) are repeated and the repeated sequences are next to each
other. By identifying repeats of a specific sequence at specific locations in the genome, it’s possible
to create a genetic profile of an individual. STR analysis is the most common method of determining
genetic profiles in forensic cases.
Single Nucleotide Polymorphism (SNP): A variance in the DNA sequence that occurs when
a single nucleotide in the genome differs between paired chromosomes in an individual. When taken
from a hypervariable region of the genome, a SNP can be useful in DNA identification. However,
the power of discrimination is not as great, making their analysis more controversial in forensics.
Low Copy Number (LCN): DNA profiling technique that involves a greater amount of copying
(amplification) from a smaller amount of sample. A DNA profile could be obtained from only a few
cells of skin or sweat left from a fingerprint. The technique is not used in the United States.
CODIS: The Combined DNA Index System (CODIS) is a computer system that stores DNA
profiles created by federal, state and local crime laboratories. Law enforcement can search the
database to assist in identifying crime suspects. The database is funded by the FBI.