DNA fingerprinting is a forensic technique used to identify people by comparing highly variable regions of their DNA. It matters because tiny biological traces such as blood, saliva, hair roots, or skin cells can link a suspect to a crime scene or help exclude an innocent person. Modern forensic DNA work focuses on short tandem repeats, or STRs, which vary greatly among individuals.
A DNA profile is powerful because it uses many STR locations together, making a random match very unlikely.
Understanding DNA Fingerprinting
A laboratory does not read a person’s entire genome for a forensic profile. It measures the length of selected repeated sections at known locations. Each version at a location is called an allele.
Since people usually inherit one allele from each biological parent, a profile often shows two allele lengths at one location. After testing, DNA fragments move through a thin capillary tube under an electric field. Smaller fragments travel differently from larger ones.
Fluorescent labels make the fragments visible as colored peaks on a computer graph. The peak positions identify allele lengths. The peak heights give clues about how much DNA was detected.
PCR is useful when a sample contains very little DNA, but it can introduce limits. The copying process runs through repeated heating and cooling steps. In each cycle, the amount of target DNA can roughly double under ideal conditions.
Real samples are rarely ideal. DNA may be broken down by heat, moisture, sunlight, or bacteria. Some chemicals can block the enzymes used in PCR.
When starting DNA is scarce, one allele may copy less successfully than the other. This is called allele dropout.
A true contributor can then appear to have only one allele at a location. Laboratories use controls, repeat tests, and minimum peak rules to reduce the chance of treating weak signals as certain results.
Many crime scene samples contain DNA from more than one person. A mixed profile can show more than two alleles at a location. Analysts must decide whether peaks come from a main contributor, a smaller contributor, or background effects such as stutter.
Stutter is a small byproduct of PCR that is usually one repeat shorter than a real allele. It can resemble a weak contributor if it is not recognized. This is why DNA interpretation requires trained analysts and careful records.
A match statement should describe the strength of the evidence, not claim that the evidence proves a person was present. Match probabilities depend on allele frequencies in relevant population groups. They also rely on assumptions about how genetic variants are inherited and counted.
CODIS is a system that allows approved laboratories to compare profiles using shared core locations. A search can connect an unknown profile with a profile already held in a database, but a database hit is a lead rather than a final conclusion. The original sample must be checked, and investigators need supporting evidence.
Close biological relatives share more DNA than unrelated people, so a relative can sometimes produce a partial association. Students should separate three ideas when studying this topic. A laboratory profile is a pattern of measured DNA fragments.
A statistical result describes how unusual that pattern is. A legal conclusion depends on the full case, including collection methods, contamination controls, and whether the sample could have arrived by innocent contact.
Key Facts
- STR stands for short tandem repeat, a repeated DNA sequence such as AGATAGATAGAT at a specific locus.
- A forensic DNA profile usually compares 13 or more STR loci, and modern CODIS core profiles use 20 loci in the United States.
- PCR copies small amounts of DNA so that trace samples can be analyzed: one DNA target can become about 2^n copies after n PCR cycles.
- Match probability across independent loci is multiplied: P(total match) = P1 x P2 x P3 x ... x Pn.
- A person has two alleles at most STR loci, one inherited from each biological parent.
- DNA evidence can include people or exclude people, but it must be interpreted with contamination risk, sample quality, and population statistics in mind.
Vocabulary
- DNA fingerprinting
- DNA fingerprinting is the process of identifying or comparing individuals using patterns of genetic variation in their DNA.
- Short tandem repeat
- A short tandem repeat is a short DNA sequence repeated multiple times in a row at a specific chromosome location.
- PCR
- PCR is a laboratory method that makes many copies of a selected DNA region from a small starting sample.
- CODIS
- CODIS is a forensic DNA database system used to store and compare DNA profiles from crime scenes, convicted offenders, arrestees, and missing persons cases.
- Allele
- An allele is a version of a genetic marker, such as a specific number of repeats at an STR locus.
Common Mistakes to Avoid
- Treating a DNA match as absolute proof of guilt is wrong because DNA can show contact or biological association, but it does not by itself prove when, why, or how the DNA was left.
- Ignoring contamination controls is wrong because trace DNA can be transferred by handling, lab error, or mixed samples, which can affect interpretation.
- Adding match probabilities instead of multiplying them is wrong because independent STR loci are combined by multiplication to estimate an overall random match probability.
- Assuming all DNA tests sequence the whole genome is wrong because standard forensic profiles usually examine selected STR markers, not every base in a person's DNA.
Practice Questions
- 1 A PCR test starts with 8 copies of a DNA target. After 25 cycles, how many copies are expected if each cycle doubles the target?
- 2 Three independent STR loci have random match probabilities of 1/10, 1/50, and 1/100. What is the combined random match probability?
- 3 A partial DNA profile from a crime scene matches a suspect at several loci, but the sample is a mixture from at least two people. Explain why investigators must be careful before treating the match as strong evidence.