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Biology
DNA Fingerprinting
STRs, CODIS, and Forensic Matching
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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.
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.