How does a forensic DNA profile work, and what do labs actually read?
A DNA profile is not your genome: it is a list of about twenty pairs of numbers, each one a count of repeats.
Guess first
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Which one is it?
A forensic lab does not read your whole genome. It counts. At about twenty chosen spots in your DNA, it measures how many times a short sequence of letters repeats in a row, and the list of numbers that comes out is your DNA profile.
About 99.9% of human DNA is the same in every person, so the lab goes where people differ. Those places are short tandem repeats: a few DNA letters, such as GTC, copied over and over, typically 5 to 50 times. Because the copying machinery can slip on repetitive text, skipping or adding a copy, the count varies a lot from person to person.
Each tested spot is called a . At every locus you carry two repeat counts, one inherited from each parent. For example, a profile might read 12 and 15 at one locus and 9 and 9 at the next. Two equal counts make you at that spot; two different counts make you heterozygous.
To read the counts, the lab copies those stretches many times over with , a method invented in 1983 by Kary Mullis, then sorts the copies by size, because more repeats means a longer piece of DNA. Since PCR was invented, even samples can be analysed.
Step 1: Take a sample
A trace of DNA from a swab or an object.
Step 2: Copy it with
The chosen repeat stretches are copied again and again.
Step 3: Sort by size
An electric field separates the pieces: longer ones move slower.
Step 4: Read the counts
Two repeat numbers at each form the profile.
North America almost universally uses the 20 CODIS core loci, the United Kingdom 17, and Australia 18. The US core was 13 loci until 2017, when seven more were added.
Try it
Longer repeat, longer fragment
y = 4x + 100
Challenge · Set 4 letters per repeat and 100 other letters. How long is a fragment with 10 repeats?
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Recap
A DNA profile counts repeats at a fixed list of spots, one number from each parent per spot.
💡 A trick to remember it · Think of a stutter in a song: everyone's DNA repeats a few words, and the lab just counts how many times, spot by spot.
Surprising fact · The first microsatellite was characterised in 1984 in Leicester, and the method later helped identify Josef Mengele's remains.
Sources (3)
No source, no claim. Every fact in this lesson (14 claims) cites at least one of these.