Applications at Higher Level: The same statement as SL. HL papers lean on the reasoning: why profiles use many markers, and what a match does and does not prove.
Practise this as you read
- Deduce the father or the suspect from a set of profiles, with a reason.
- Explain why many markers reduce the chance of a false match.
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The big idea: DNA profiling (also called DNA fingerprinting) reads the parts of a person's DNA that differ the most from one individual to the next, producing a unique pattern.
Two main stages
PCR
Copies the chosen DNA millions of times (because a sample is usually tiny).
Gel electrophoresis
Separates those copies by size, giving a pattern of bands that is (almost) unique to each person.
- DNA profiling
- A technique that produces a pattern of DNA fragments unique to an individual, used to identify people or test relationships.
- PCR (polymerase chain reaction)
- A laboratory method that makes millions of copies of a chosen piece of DNA — the amplification stage of profiling.
- Amplification
- Making many copies of a piece of DNA so there is enough to analyse.
- Gel electrophoresis
- A technique that separates DNA fragments by size by pulling them through a gel with an electric field.
- Fragment
- A short piece of DNA. People differ in how long their fragments are, which is what profiling detects.
| Stage | What it does | Why it is needed |
|---|---|---|
| 1. PCR (amplification) | Makes millions of copies of the chosen DNA fragments | A crime-scene or test sample is usually far too small to analyse — PCR multiplies it up |
| 2. Gel electrophoresis | Separates the DNA fragments by size | Different people have fragments of different lengths — separating them reveals each person's unique pattern of bands |
| 3. Read the profile | Compares the band pattern between samples | Matching patterns suggest the samples came from the same person (or close relatives) |
Copy first, then separate: The order matters: you amplify first (PCR makes enough DNA to see), then you separate (gel electrophoresis spreads the fragments out by size).
PCR is about quantity; gel electrophoresis is about sorting.
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Every nucleated body cell of an organism carries a complete copy of the whole genome — a skin cell holds exactly the same genome as a liver cell.
Why a tiny sample is enough: That is why a single cell from almost anywhere in the body can supply a full copy of the genome for analysis, and why a tiny sample (a drop of blood, a hair root, a cheek swab) is enough for DNA profiling.
Why profiling looks at the VARIABLE regions: Most of the genome is almost identical from person to person — that is why we are all human. So the coding genes are no use for telling individuals apart.
How a DNA profile is built
Find the repeats
DNA profiling looks at short stretches of non-coding DNA that are repeated over and over — called short tandem repeats.
Count them
The number of repeats at each place varies a lot between individuals.
Build the pattern
By counting the repeats at several places in the genome, you build a pattern — a DNA profile — that is essentially unique to each person (identical twins aside).
What a profile can show: Because half of your DNA comes from each parent, your profile shares repeat patterns with your relatives.
So comparing profiles can:
• match a sample to the individual it came from (forensics);
• test whether two people are related (for example a parentage test);
• the more repeat patterns two profiles share, the more closely related the two individuals are.
| Feature | Coding regions (genes) | Short tandem repeats (used for profiling) |
|---|---|---|
| What they do | Code for proteins / products | Do not code for a product (non-coding) |
| How alike between people | Very similar — most people share them | Highly variable — differ a lot between individuals |
| Useful for telling people apart? | No — too similar to distinguish | Yes — the variation is the whole point |
| What a profile compares | Not used | The number of repeats at several places in the genome |
The genome
- ALL of an organism's DNA
- The same complete copy in every nucleated cell
- Mostly shared between members of a species
- Contains genes and non-coding DNA
A DNA profile
- Reads only the variable repeat regions
- Differs between individuals
- Used to identify a person or test relatedness
- More shared patterns = more closely related
An easy way to remember: The genome is what makes us the same (mostly identical DNA). DNA profiling zooms in on the few variable repeat regions that make each of us different.
The big idea: DNA profiling compares band patterns. A child shares bands with both parents; a crime-scene sample matches only one person.
The points to remember
- Paternity: every band in the child comes from the mother or the father.
- The child's bands that are not in the mother's profile must all be in the father's.
- Forensics: a suspect matches when every band of the crime-scene sample is in their profile.
- Unrelated people can share a few bands by chance.
- So many markers are compared: each extra marker makes a false match much less likely.
Remember it as: Child = half mum, half dad. Match = every band.
Real example: 20 markers. Since 2017 the FBI's national DNA database in the USA has compared 20 core markers in every profile, up from 13, to make a false match even less likely.
Say bands, not genes: Profiles compare bands (repeated sequences), not genes. Name the man who has all the child's non-maternal bands, then give that reason.
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Far more than crime scenes: Anywhere a tiny amount of DNA must be found, copied or compared, PCR and gels are used.
Medicine
- Detecting the DNA of a virus or bacterium in a patient's sample.
- Testing for alleles that cause genetic diseases.
Food and wildlife
- Checking which species is in meat or fish products.
- Tracing illegal ivory or bushmeat to its species.
Research
- Studying ancient DNA from bones and teeth.
- Comparing DNA between species to see how they are related.
Real example: COVID-19 tests. During the COVID-19 pandemic, laboratory tests copied the genetic material of SARS-CoV-2 from nose and throat swabs by PCR, so even tiny amounts of the virus could be detected.
The two jobs, every time
- PCR is used whenever there is too little DNA to study: a hair, a swab, an old bone.
- Gel electrophoresis then shows which fragments are there, by their size.
How it is asked: Data questions show profiles: name the techniques (PCR, gel electrophoresis), deduce the father or the suspect with a reason, and explain why many markers are used.
Explain the reasons for using many different markers to produce a DNA profile in forensic investigations.
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