What does “a one in a billion DNA match” actually mean?
“One in a billion” sounds like proof, but it measures how rare a profile is, not how likely a suspect is to be guilty.
Guess first
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It means this: if you picked a stranger at random from the population, the chance that their DNA profile would match the one from the crime scene purely by coincidence is one in a billion. It is a statement about coincidence. It is not a statement about guilt. English courts spell the difference out: judges must make sure that the jury does not confuse the match probability, the chance that a random person has a matching profile, with the probability that a person with matching DNA committed the crime.
Where does a number like one in a billion come from? A profile is a row of repeat counts at about twenty spots, so the lab can use the : the chance of the whole profile is the chance at the first spot, times the chance at the second, and so on. Multiplying twenty small chances produces enormous odds. The result has been match probabilities of 1 in a or more.
Real life adds footnotes. Identical twins share a profile. With the older technique, the theoretical -match risk of 1 in 100 billion became, in practice, about 1 in 1,000, because identical twins are 0.2% of the human population. Laboratory error is almost certainly more likely than a coincidence. A , from a damaged sample, has a larger random-match probability than a full one. And in the early days, the population data needed to compute the numbers accurately was sometimes missing, so for a few years ceilings were put on the figures, controversially.
So read a huge number carefully. It tells you how rare a profile is, not how likely it is that this person did it.
Try it
How fast the odds multiply
y = 1 · 4ˣ
Challenge · If each spot is shared by 1 person in 10, how many spots make a coincidental match 1 in a million?
Quiz me
0/3
Recap
A one-in-a-billion match tells you how rare the profile is, not how likely the suspect is to be guilty.
💡 A trick to remember it · The number is a rarity label, like on a stamp: it tells you how scarce the profile is, not who put it there.
Surprising fact · With the older RFLP technique, a theoretical risk of 1 in 100 billion became about 1 in 1,000 in practice, because identical twins are 0.2% of people.
Connects to
- 👻 Can DNA at a crime scene point to a killer who does not exist?
- 🧬 How does a forensic DNA profile work, and what do labs actually read?
- ⚖️ What is the prosecutor's fallacy, and how did it affect the Sally Clark case?
- 🧬 How did one unexpected X-ray image lead to DNA fingerprinting?
- 🎲 How did an unfinished game of chance give birth to probability theory?
- Conditional probability
Sources (3)
No source, no claim. Every fact in this lesson (13 claims) cites at least one of these.