Forensic science relies heavily on DNA evidence to connect individuals to a crime scene or to exclude them from an investigation. Although the vast majority of human genetic material is identical between any two people, the small percentage of difference provides the basis for identification. This difference is found in variations called alleles, which are alternative forms of a gene or a specific genetic marker. The power of forensic DNA analysis lies in explaining how these variations, when examined in combination, create a unique genetic blueprint for nearly every individual.
Alleles: The Basis of Individual Genetic Variation
An allele represents one of two or more possible versions of a genetic sequence found at a specific location, or locus, on a chromosome. Since humans inherit one set of chromosomes from each parent, every person possesses two alleles for each locus.
Although human DNA is over 99.9% identical across the population, the remaining fraction of a percent contains the diversity necessary for identification. Forensic science focuses on these highly variable regions, as the combination of alleles an individual carries is distinct, forming the foundation of a unique identity.
Targeting Non-Coding Regions: Short Tandem Repeats (STRs)
Forensic scientists focus on non-coding sections of the genome that contain highly repetitive sequences known as Short Tandem Repeats (STRs). They avoid analyzing DNA that codes for physical traits or diseases, as this information is irrelevant to identification. STRs are the specific markers used in forensic analysis because they vary significantly between people.
An STR is a short DNA sequence, usually consisting of two to six base pairs, that is repeated multiple times at a particular locus. The allele at an STR locus is defined by the exact number of times this core sequence is repeated. For instance, one person might have 10 repeats, while another has 14, and these numbers are their inherited alleles for that location. The high degree of polymorphism makes STRs useful for distinguishing between individuals.
Assembling the DNA Profile for Identification
The practical application of STR alleles involves analyzing multiple loci simultaneously to create a comprehensive DNA profile. Since every person inherits one allele from each parent, the combination of these two repeat numbers defines the genotype at that specific location. For example, a person might have the allele combination “10, 14” at one locus and “6, 9” at another.
Forensic laboratories analyze a standard set of these STR loci, which in the United States currently includes 20 core markers for uploading to the national database, the Combined DNA Index System (CODIS). The full profile is the complete collection of genotypes across all 20 loci, creating a multi-locus genetic fingerprint. When a sample profile from a crime scene matches a profile from a suspect or a database, it means the two samples share the same alleles across all 20 analyzed locations.
The Statistical Certainty of Allele Combinations
Alleles are helpful to forensic science because the combination of many independent markers generates statistical confidence in an identification. While many people might share the same pair of alleles at a single STR locus, the probability of two unrelated individuals sharing the same pair of alleles across all 20 loci is extremely low. This is calculated using the Product Rule, which involves multiplying the individual frequency of each allele pair across all the analyzed loci.
Because each locus is inherited independently, the combined frequency is the product of the probabilities for all 20 markers. This mathematical principle drives the overall probability of a random match to astronomical figures. The Random Match Probability (RMP) for a full 20-locus profile is typically less than one in a trillion. This power of discrimination, derived from the unique combination of alleles, provides the scientific foundation for using DNA profiles as reliable evidence in legal proceedings.