Understanding Genetic Fingerprinting (Part 1/3) Revision Notes | A Level AQA Biology

1. Why No Two People Share the Same Genetic Fingerprint

Every human genome contains billions of base pairs, but buried within the non-coding regions are short sequences that repeat themselves over and over — and the number of times they repeat differs from person to person. This variation is the foundation of genetic fingerprinting, one of the most powerful tools in modern biology.

Think of it like a barcode. Two barcodes might look similar at a glance, but the precise pattern of lines makes each one unique. Your genome carries its own biological barcode, written in repeating DNA sequences called variable number tandem repeats, or VNTRs.

VNTRs are found at multiple loci scattered across the genome. At each locus, one individual might carry 4 repeats of a core sequence, while another carries 17. Because humans are diploid, each locus produces two alleles — one from each homologous chromosome — and both are inherited independently. The combination of allele lengths across many loci produces a pattern that is statistically unique to each individual.

The probability of two unrelated individuals sharing identical VNTR profiles across multiple loci is vanishingly small — often quoted as less than one in a billion. This is why genetic fingerprinting can be used as a reliable identifier in forensic, medical, and ecological contexts.

One subtlety worth noting: identical twins share the same DNA sequence, so their VNTR profiles are identical. This is a genuine limitation of the technique — genetic fingerprinting cannot distinguish between monozygotic twins. For all other individuals, including dizygotic twins, the profiles will differ.

2. The Genetic Fingerprinting Process: From Sample to Profile

Now that you understand what VNTRs are, the next question is: how do scientists actually reveal these repeat patterns from a tiny biological sample? The answer involves a carefully sequenced set of molecular techniques, starting with amplification and ending with a visible banding pattern.

The process begins with extracting DNA from a sample — this could be blood, saliva, hair roots, or any nucleated cell. The quantity of DNA available is often extremely small, so the first critical step is amplification using the polymerase chain reaction (PCR).

Worked example: The Genetic Fingerprinting Process: Stage by Stage

Describe and explain the sequence of steps used to produce a genetic fingerprint from a biological sample.

1. Cells are lysed (broken open) to release DNA. The DNA is then purified to remove proteins and other cellular debris. — Any nucleated cell can provide DNA. Even degraded samples can sometimes yield enough material for PCR amplification.
2. Specific primers complementary to sequences flanking the VNTR loci are added. PCR cycles (denaturation at ~95°C, annealing at ~55°C, extension at ~72°C) are repeated 25–35 times, exponentially amplifying the target regions. — PCR is essential because the original sample may contain only nanogram quantities of DNA — far too little to analyse directly. After PCR, there are millions of copies of each VNTR region.
3. The amplified fragments are loaded into wells in an agarose gel. An electric current is applied. Smaller fragments migrate further through the gel in a given time; larger fragments migrate less far. — DNA is negatively charged, so it moves towards the positive electrode. The gel acts as a molecular sieve, separating fragments by size. Each VNTR allele produces a band at a position corresponding to its length.
4. Fragments are stained (e.g. with ethidium bromide or a fluorescent dye) or detected using labelled probes. The resulting pattern of bands — the genetic fingerprint — is photographed or digitally recorded. — Each individual produces a unique pattern of bands across multiple VNTR loci. The position of each band corresponds to the number of repeats at that locus. Comparing band patterns between individuals reveals genetic relationships.

Answer: The process runs: DNA extraction → PCR amplification of VNTR regions → gel electrophoresis to separate fragments by size → visualisation of the banding pattern. The resulting profile is unique to each individual (except identical twins).

The banding pattern produced is the genetic fingerprint itself. Each band represents a VNTR allele of a particular length. Because you inherit one allele at each locus from each parent, you will share approximately 50% of your bands with each biological parent — a fact that underpins paternity and maternity testing.

3. Determining Genetic Relationships Using Genetic Fingerprinting

With the process established, consider what the banding pattern actually tells us about biological relationships. This is where genetic fingerprinting moves from a laboratory technique to a genuinely powerful analytical tool.

Because each individual inherits one allele at every VNTR locus from each biological parent, approximately half of a child's bands should match those of the biological mother and the other half should match those of the biological father. However, if a parent is homozygous at a locus (has two alleles of the same length), the child may share that band with both parents, so the 50/50 split is not always exact. If a putative father's profile shares approximately 50% of bands with a child — after accounting for the bands already explained by the mother — this is strong evidence of biological paternity.

The more VNTR loci are analysed, the more statistically robust the conclusion becomes. Analysing ten or more loci reduces the probability of a coincidental match to negligible levels, which is why modern forensic and clinical protocols use multiple loci simultaneously.

Genetic fingerprinting is also used in forensic science to match biological material found at a crime scene to a suspect. A DNA sample from the scene is amplified by PCR and its banding pattern compared to that of a reference sample from the suspect. A match across multiple loci provides very strong evidence of a common source, though courts require careful statistical interpretation of the probability of a coincidental match.

In summary, genetic fingerprinting determines genetic relationships by comparing the number and position of VNTR-derived bands between individuals — a direct application of the principle that VNTR profiles are inherited in a predictable Mendelian fashion.

4. Assessing Genetic Variability Within a Population

Beyond identifying individuals and relationships, genetic fingerprinting has a broader ecological and conservation application: measuring the genetic variability of a population. This connects directly to the earlier point about VNTRs being highly polymorphic — the more variation in VNTR profiles across individuals in a population, the greater the genetic diversity.

Genetic variability matters because populations with low genetic diversity are more vulnerable to disease, environmental change, and inbreeding depression. Conservation biologists use genetic fingerprinting to assess whether a threatened population retains sufficient genetic diversity to remain viable, and to guide breeding programmes that maximise diversity.

In practice, researchers collect DNA samples from multiple individuals across a population, generate genetic fingerprints for each, and then compare the profiles. A population with high variability will show many different band patterns across individuals. A population with low variability — perhaps due to a historical bottleneck or founder effect — will show many shared bands, indicating that individuals are closely related and allele diversity is limited.

Genetic fingerprinting data can also be used to calculate indices of genetic diversity, such as the proportion of polymorphic loci or heterozygosity values. These quantitative measures allow researchers to compare variability between populations or track changes in diversity over time — for example, monitoring whether a captive breeding programme is successfully maintaining genetic diversity before reintroduction to the wild.

Assessing genetic variability through VNTR analysis therefore provides both a snapshot of current diversity and a tool for managing populations — a direct application of the principle that the probability of two individuals sharing identical VNTR profiles is very low in genetically diverse populations, but rises as diversity falls.

5. Exam Technique: Answering Genetic Fingerprinting Questions

Genetic fingerprinting questions appear across a range of command words in AQA A Level papers. The most common errors come not from misunderstanding the biology, but from imprecise language and incomplete reasoning chains. The guidance below addresses the patterns that most frequently cost marks.

For extended-response questions asking you to evaluate genetic fingerprinting as a technique, structure your answer to address both its strengths (high discriminatory power across multiple loci, applicability to tiny or degraded samples via PCR, quantitative data on population diversity) and its limitations (inability to distinguish identical twins, requirement for reference samples in forensic contexts, potential for contamination of samples). A conclusion that weighs these points against the specific context of the question will secure the highest mark band.

This calculation reinforces the core AQA specification requirement: the probability of two individuals having the same VNTRs is very low, and that probability decreases further as more loci are included in the analysis. In exam answers, quoting this principle with reference to multiple independent loci will always strengthen a response.