The 1 in a quintillion math that identifies a single human being
We share 99.9% of our DNA with every other person on Earth. Yet, through the power of repetitive genetic sequences, scientists can now calculate match probabilities so precise they can distinguish one individual from a crowd of a quintillion.
The breakthrough arrived in 1984 at the University of Leicester, where geneticist Sir Alec Jeffreys discovered that certain regions of DNA contain highly variable, repetitive sequences. He termed these unique patterns "genetic fingerprints." While 99.9% of human DNA is identical across the species, these specific areas—known as variable number tandem repeats (VNTRs) or short tandem repeats (STRs)—vary significantly between individuals, with the sole exception of monozyg/zygotic twins.
The practical power of this technology was proven during a landmark criminal investigation in the mid-1980s. Following the murders of two teenagers in Narborough, Leicestershire, investigators used DNA from blood samples to screen approximately 5,000 local men. This process not only exonerated an initial suspect, Richard Buckland, but led to the 1988 conviction of Colin Pitchfork. The case was particularly notable because Pitchfork had attempted to use a coworker, Ian Kelly, to provide a fraudulent sample via a forged passport.
Modern forensics relies heavily on Polymerase Chain Reaction (PCR), a technique developed by Kary Mullis in 1983. PCR allows scientists to amplify tiny, even degraded, fragments of DNA by cycling through denaturation, annealing, and extension. By targeting specific STR loci—such as the 20 core loci used in the North American CODIS system—analysts can apply the product rule. Because these loci are independently assorted, multiplying their individual probabilities can generate a statistical certainty of 1 in 10^18, making the genetic signature virtually unique to one person.
Source: DNA profiling
