SPEC 3.8.4.2–3.8.4.3 - DNA PROBES & GENETIC FINGERPRINTING
DNA Probes, Genetic Screening & Fingerprinting
Labelled Probes, Hybridisation, Personalised Medicine, VNTRs & Gel Electrophoresis
DNA Probes
Key Definition
DNA probe: A short, single-stranded DNA sequence that is complementary to a target sequence. It is labelled — either with a radioactive isotope or a fluorescent dye — so that it can be detected once it has bound to the target.
How Probes Work — DNA Hybridisation
When a labelled probe is added to single-stranded DNA from a patient, it will only bind (hybridise) to a sequence that is complementary to it. Hydrogen bonds form between the probe and the target sequence.
If the target allele is present → probe binds → signal detected
If the target allele is absent → probe does not bind → no signal
Radioactive Label
Probe carries a radioactive isotope. After hybridisation and washing, X-ray film placed over the membrane produces a dark band at the position of the probe — this is autoradiography.
Fluorescent Label
Probe carries a fluorescent dye. After hybridisation and washing, the membrane is illuminated with UV light — a fluorescent signal shows where the probe has bound.
Genetic Screening with DNA Probes
DNA probes can be used to screen patients for specific alleles associated with heritable conditions, drug responses, or health risks.
1
Obtain a DNA sample from the patient (e.g. blood, saliva)
2
Denature the DNA to produce single-stranded fragments (e.g. by heating)
3
Add the labelled DNA probe — it hybridises to the complementary target allele if present
4
Wash away unbound probe
5
Detect bound probe by autoradiography (radioactive label) or UV illumination (fluorescent label) — a signal confirms the target allele is present
Key Examples
Probes can detect: alleles associated with heritable conditions (e.g. cystic fibrosis allele, BRCA1/2 variants); alleles affecting drug metabolism (pharmacogenomics); alleles indicating disease susceptibility.
Personalised Medicine & Genetic Counselling
Key Definitions
Personalised medicine: Tailoring medical treatment to an individual's genetic profile — for example, choosing a drug that will be correctly metabolised by that patient based on their alleles.
Genetic counselling: Advising individuals and families about the significance of genetic screening results, their risk of developing conditions or passing them to offspring, and the options available to them.
Pharmacogenomics
Different alleles of genes coding for drug-metabolising enzymes mean individuals vary in how they process drugs. Screening identifies the relevant allele → correct drug and dose prescribed → improved efficacy, reduced side effects.
Evaluating Screening
Benefits: early diagnosis, preventive action, informed family planning, targeted treatment
Concerns: psychological impact of positive result; risk of genetic discrimination (insurance, employment); false positives/negatives; privacy of genetic data
Exam Tip
AQA requires you to evaluate genetic screening — present both benefits AND concerns. Screening identifies risk, not certainty — carrying a disease allele does not always mean the condition will develop.
Variable Number Tandem Repeats (VNTRs)
Key Definition
Variable number tandem repeat (VNTR): A region of non-coding DNA where a short sequence is repeated a variable number of times at a specific locus. The number of repeats varies greatly between individuals.
Why VNTRs are Useful
Every individual has their own unique combination of VNTR lengths across the genome. The probability of two individuals having identical VNTR profiles is extremely low — making VNTRs the basis of genetic fingerprinting.
VNTRs are in non-coding regions — variation has no effect on phenotype
Every person inherits one set of VNTR alleles from each parent
The only exception: identical twins have the same VNTR profile
Important
VNTR profiles are not truly unique to an individual — they are extremely unlikely to be shared between unrelated individuals. This is why results are presented as probability statements, not certainties.
Genetic Fingerprinting — The Process
1
Extract DNA from a sample — blood, hair root, saliva, sperm or other tissue
2
PCR amplifies the VNTR regions using primers that flank each VNTR locus — the length of each PCR product directly reflects the number of repeats at that locus
3
Gel electrophoresis separates the amplified fragments by size — fragments move through the gel toward the positive electrode; smaller fragments travel further
4
Visualise the banding pattern — the resulting pattern of bands is the individual's genetic fingerprint
Key Point
The position of each band reflects the size of the fragment at that VNTR locus. More repeats → longer fragment → band sits closer to the well (travels less far).
Reading Gel Electrophoresis Results
DNA fragments are negatively charged (due to the phosphate backbone) and migrate toward the positive electrode when a current is applied.
Schematic Gel — Forensic Example
CRIME SCENE
SUSPECT A
SUSPECT B
↑ Well (top) — fragments run toward positive electrode (bottom) ↓ — smaller fragments travel further
Interpreting Results
Suspect A's band pattern matches the crime scene — consistent with their DNA being at the scene. Suspect B's pattern does not match — they can be excluded.
Examiner Alert
A DNA match does not prove guilt — it indicates the DNA is consistent with being from that individual. Results are stated as probability. Identical twins have matching profiles. Never state a match "proves" the crime was committed.
Applications of Genetic Fingerprinting
ApplicationHow it is Used
Forensic scienceMatch DNA from crime scene samples (blood, hair, saliva, sperm) to a suspect; or exclude innocent individuals from investigation
Paternity / relationship testingA child's bands each originate from one biological parent — all of a child's bands should appear in the profile of the biological mother or father. Used to confirm or deny biological relationships.
Medical diagnosisIdentify specific bacterial or viral strains causing infection; confirm disease diagnosis; identify individuals at risk of heritable conditions
Animal and plant breedingDetermine genetic diversity within captive populations (conservation); confirm pedigree; identify individuals; avoid inbreeding; select desirable traits
Population geneticsDetermine genetic variability within and between populations; study evolutionary relationships
Summary
DNA Probes & Hybridisation
Short, single-stranded, labelled (radioactive or fluorescent) DNA complementary to target allele
Hybridises to complementary target in denatured patient DNA → detected by autoradiography or UV
Applications: heritable condition screening, drug response (pharmacogenomics), health risk identification
Personalised Medicine & Genetic Counselling
Personalised medicine: tailor treatment to individual's genetic profile
Genetic counselling: inform patients/families about genetic risk and options
Evaluate: benefits (early diagnosis, targeted treatment) vs concerns (psychological, discrimination, privacy)
VNTRs & Genetic Fingerprinting
VNTRs: non-coding tandem repeats; number of repeats varies between individuals; very low probability of two unrelated individuals sharing the same profile
Process: extract DNA → PCR amplifies VNTR regions (primers flank each locus; product length = repeat number) → gel electrophoresis → visualise banding pattern
Gel: DNA negatively charged → migrates to positive electrode; smaller fragments travel further
Uses: forensic science, paternity testing, medical diagnosis, animal/plant breeding, population genetics
Quick-Check Practice
Q1. Describe how a labelled DNA probe is used to detect a specific allele in a patient's DNA sample. [4 marks]
The patient's DNA is denatured to produce single-stranded fragments (1); the labelled probe (complementary to the target allele) is added and hybridises / hydrogen bonds form with the complementary target sequence if present (1); unbound probe is washed away (1); the position of the bound probe is detected by autoradiography (radioactive label) or UV illumination (fluorescent label) (1).
Q2. Explain what VNTRs are and why they are used in genetic fingerprinting. [3 marks]
VNTRs (variable number tandem repeats) are regions of non-coding DNA where a short sequence is repeated a variable number of times (1); the number of repeats varies between individuals (1); the probability of two unrelated individuals having the same combination of VNTR lengths across the genome is very low, producing a unique banding pattern (1).
Q3. In a forensic investigation, gel electrophoresis shows that the band pattern of the crime scene DNA matches Suspect X perfectly. A student concludes: "This proves Suspect X committed the crime." Evaluate this conclusion. [2 marks]
The conclusion is not valid (1); a DNA match shows the DNA is consistent with coming from Suspect X but does not prove guilt — results are probabilistic, DNA could have been left at the scene innocently, and identical twins would produce the same profile (1).
Q4. State two different applications of genetic fingerprinting outside of forensic science. [2 marks]
Any two from: paternity/maternity/relationship testing (1); medical diagnosis — identifying bacterial/viral strains or diagnosing heritable conditions (1); animal or plant breeding — determining genetic diversity, confirming pedigree, avoiding inbreeding (1); determining genetic variability within a population (1).
Next: DNA Probes