Reading a DNA profile in an exam is a logic puzzle, not a biology recall question. There is one rule — every band in a child’s profile must come from the mother or the father — and once you apply it methodically, the answer falls out. This page teaches the technique and then the reading.
📘 What you need to know
DNA profiling (genetic fingerprinting) lets scientists identify suspects of a crime, identify victims, and establish parentage.
The regions analysed are VNTRs — Variable Number Tandem Repeats.
VNTRs are repeating, short, non-coding regions, roughly 20 to 50 bases long.
Every person except an identical twin has a unique combination of VNTRs.
Profiling uses PCR to amplify, restriction endonucleases to cut, and gel electrophoresis to separate by length.
VNTRs are inherited from both parents, so a parent shares about 50% of the offspring’s bands.
NOS: reliability is enhanced by increasing the number of measurements — more VNTR markers means a lower chance of a false match.
What is actually being compared
DNA profiling does not read your whole genome. It looks at VNTRs: short, non-coding stretches of DNA that repeat, and where the number of repeats varies from person to person.
Each VNTR region is between about 20 and 50 bases long.
Because the number of repeats differs, the fragment lengths differ, so the bands land at different heights on a gel.
Everyone except identical twins has a combination of VNTRs that is unique to them.
Gel electrophoresis separates those VNTR fragments according to length, creating a pattern of bands that is unique to every individual.
🧩 Making a DNA profile, in order
Extract DNA from a sample — the root of a hair, a spot of blood, semen or saliva.
Amplify it using PCR, producing many copies from a very small sample.
Cut the amplified DNA into fragments using restriction endonucleases.
Separate the fragments by mass using gel electrophoresis.
Add radioactive or fluorescent probes that are complementary to the VNTRs.
Use X-ray imaging or UV light to visualise where the probes are.
Analyse and compare the resulting patterns of bands.
Notice the order: amplify before you cut. There is no point cutting up a sample so small you cannot see it. PCR always comes first.
Profiling in ancestry and paternity tests
Every person inherits their VNTRs from both their mother and their father. So a profile can be used to identify someone’s parents or other ancestors.
The profile of the individual is compared with those of the potential parents.
Patterns of bands are compared. If many bands are shared, that indicates ancestry.
A parent would be expected to share about 50% of the offspring’s bands.
The rule that solves every paternity question
Every band in the child’s profile must appear in the mother’s profile, the father’s profile, or both.
If a man is missing a child band that the mother does not have either, he is not the father.
Bands 1, 3 and 7 in the child are also in the mother, so they are dead ends. Bands 4 and 6 are where the whole question is decided.
WORKED EXAMPLE
Use the DNA profiles above to determine which of the four men is the child’s father. [4]
Step 1: list the child’s bands
The child has bands at positions 1, 3, 4, 6 and 7.
Step 2: cross off any the mother also hasThe mother has 1, 3 and 7, so those could all have come from her. They cannot eliminate anyone.Step 3: the mother does not have band 4, so it must be paternalOnly A and B have band 4, so C and D are eliminated.Step 4: the mother does not have band 6 eitherOnly B and D have band 6, and D is already out, so A is eliminated too.B is the father — he is the only man with both band 4 and band 6work down the child’s lane in order and eliminate. Never start from the men’s lanes.
Profiling in forensic investigations
Forensic scientists use DNA profiling to identify suspects:
Samples of body cells or fluids — blood, saliva, hair, semen — are taken from the crime scene or the victim’s body.
The DNA is removed and profiled.
The profile is compared with samples from the suspect (or from a criminal DNA database), from the victim, and from people with no connection to the crime, which act as control samples.
Great care must be taken to avoid contamination of the samples.
It is also used to identify victims who are otherwise unidentifiable, and — just as importantly — to eliminate innocent people whose DNA happens to be at the scene.
A single band that a suspect has but the scene does not is enough to rule that person out. Suspects 1 and 2 each have one.
WORKED EXAMPLE
Use the forensic gel above to decide which suspect was most likely present at the crime scene, and explain your reasoning. [3]
Step 1: account for the victim firstThe victim’s bands at 2, 5 and 8 explain three of the seven crime scene bands.Step 2: check each suspect against the remaining bands (1, 3, 4 and 7)Suspects 1 and 2 each have a band at position 6, which does not appear at the scene at all — so neither of them can be the source.Step 3: check suspect 3Suspect 3 has bands at 1, 3, 4 and 7 — exactly the bands left over.Suspect 3 is most likely to have been present, as the victim and suspect 3 together account for every bandsay “most likely”, not “definitely”. A profile is strong evidence, not proof.
NOS: reliability comes from more measurements
Increasing the reliability of scientific conclusions matters everywhere, but it matters most when the evidence is going to be used in a court of law. It is almost impossible to be 100% certain that any evidence is absolutely correct.
In DNA profiling, reliability is increased by increasing the number of VNTR markers observed and compared.
The more VNTRs used, the lower the chance of a false match.
The reasoning is worth seeing. If two unrelated people happened to share one VNTR marker, that would be unremarkable. Sharing ten by coincidence is vanishingly unlikely. Each extra marker multiplies the improbability of a chance match, which is why real forensic profiles use many markers rather than the handful shown in an exam diagram.
A limitation worth quoting. Identical twins have the same VNTRs, so DNA profiling alone cannot tell them apart. It is the standard exception in this topic.
Worked example
WORKED EXAMPLE
A forensic laboratory compares six VNTR markers rather than three. Explain the advantage. [2]
Say what more markers do to a chance matchThe chance that two unrelated people share all the markers by coincidence falls with each extra marker compared.Now name the benefitThe chance of a false match is lower, so the conclusion is more reliablethe phrase “increases reliability” is what the NOS mark is for.
💡 Exam tip
For paternity questions, go down the child’s lane band by band. Ignore bands the mother also has.
For forensic questions, look for the most bands in common, and for any band a suspect has that the scene does not.
Remember the crime scene sample often contains the victim’s DNA too. Subtract it first.
Learn the seven steps of profiling in order — amplify before you cut.
Say VNTR and restriction endonuclease by name. Both are recall marks.
Write conclusions as likely rather than certain, and mention control samples if the question is about validity.
⚠ Common mix-up
Starting from the men’s lanes. Always start from the child’s bands.
Using a shared band as proof of paternity. If the mother has it too, it proves nothing.
Thinking the profile covers whole genes. VNTRs are non-coding regions.
Saying a profile is unique to everyone. Identical twins share theirs.
Expecting a parent to match every band. A parent shares about half.
Forgetting contamination. It is a standard evaluation point in forensic questions.
Confusing restriction endonucleases with DNA polymerase. One cuts DNA, the other builds it.
Up next: The Mechanism of DNA Replication — the messy detail behind the tidy picture, including why one of the two new strands has to be built backwards.
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