DNA fingerprinting

Apart from identical twins, no two people have exactly the same DNA. DNA fingerprinting turns that fact into an identity test: instead of reading someone's whole genome, it measures a small set of highly variable spots in the DNA that differ enormously from person to person, producing a pattern as individual as a fingerprint.

Modern DNA fingerprinting focuses on short tandem repeats (STRs) — short DNA motifs repeated back-to-back, where the number of repeats at a given spot varies a lot between people. At one STR location you might have 11 repeats on one chromosome and 14 on the other, while someone else has 9 and 16. PCR amplifies a panel of these locations (often around twenty), and the lengths of the amplified pieces are measured. The combination of repeat numbers across all the measured spots is the profile. Each spot alone is not unique, but the chance that two unrelated people share the same value at twenty independent spots is astronomically small, so the full profile is effectively unique.

DNA fingerprinting underpins forensic identification, paternity testing, identifying remains, and tracking lineages. It is powerful precisely because it needs only tiny amounts of DNA (amplified by PCR) and reads a few super-variable markers rather than a whole genome. But the same sensitivity demands honesty about its limits: it identifies whose DNA is present, not how or when it got there; trace amounts can be transferred innocently; mixtures of several people's DNA are hard to interpret; and identical twins share a profile. A DNA match is strong evidence of source, but it is not the same thing as proof of an act.