Genetic Fingerprinting and DNA Profiling: GCSE Biology Guide

What is genetic fingerprinting?

Genetic fingerprinting is a technique that analyses specific regions of a person's DNA where short sequences are repeated many times. These repeated regions are highly variable between individuals, which is what makes the resulting profile unique. Scientists do not read the entire genome; instead, they focus on these variable repeat regions to generate a banding pattern that acts like a biological barcode.

The technique exploits the fact that although all humans share approximately 99.9% of their DNA sequence, the number of times certain short sequences repeat differs dramatically from person to person. This variation is heritable, so children share some banding patterns with each biological parent.

What are VNTRs and STRs?

Two types of repeat sequences are used in genetic fingerprinting:

• VNTRs (Variable Number Tandem Repeats): Longer repeated units, typically 10–100 base pairs each. Earlier techniques relied on VNTRs, but they require larger DNA samples and take longer to analyse.
• STRs (Short Tandem Repeats): Shorter repeated units, typically 2–7 base pairs each. Modern forensic analysis uses STRs because they can be amplified from tiny or degraded DNA samples using the polymerase chain reaction (PCR). The UK National DNA Database uses 16 STR loci.

The statistical power of STR profiling is extraordinary: the chance of two unrelated individuals sharing the same 16-locus profile is approximately 1 in a billion.

How does the genetic fingerprinting process work?

The classic technique follows a defined sequence of steps:

1. DNA extraction. DNA is extracted from a biological sample — blood, saliva, hair roots, or skin cells.
2. Restriction digestion. Restriction enzymes cut the DNA at specific recognition sequences, producing fragments of different lengths depending on how many repeats lie between cut sites.
3. Gel electrophoresis. The DNA fragments are loaded into wells in an agarose gel and an electric current is applied. Smaller fragments travel further through the gel; larger fragments move more slowly. This separates fragments by size.
4. Southern blotting. The separated fragments are transferred from the fragile gel onto a nylon membrane. This preserves the pattern of bands.
5. Hybridisation. Radioactive or fluorescent probes — short, single-stranded DNA sequences complementary to the repeat regions — are added. They bind specifically to the VNTR or STR fragments on the membrane.
6. Autoradiograph. X-ray film (or a fluorescence detector) is placed against the membrane. The probes expose the film, revealing a pattern of dark bands. This banding pattern is the genetic fingerprint.

What are the main uses of genetic fingerprinting?

How is gel electrophoresis used to separate DNA?

Gel electrophoresis separates DNA fragments by size using an electric field. DNA carries a negative charge because of the phosphate groups in its backbone, so it migrates towards the positive electrode. An agarose gel acts as a molecular sieve: the tightly woven polymer matrix slows larger fragments more than smaller ones.

After running the gel for a set time, the fragments are visualised. A DNA ladder — a mixture of fragments of known sizes — is run alongside the samples. Comparing the distance each unknown band has travelled against the ladder allows scientists to determine fragment sizes in base pairs.

The key point for your exam: fragment size and distance travelled are inversely related — smaller fragments travel further.

Why is genetic fingerprinting more powerful than blood typing?

Blood typing divides the population into just four main groups (A, B, AB, O), so it can exclude a suspect but rarely confirm one. Genetic fingerprinting, by contrast, examines multiple loci simultaneously. Each additional locus multiplies the discriminating power exponentially. With 16 STR loci, the probability of a false match between unrelated individuals falls to around one in a billion or lower, making it one of the most powerful identification tools in forensic science.

What are the limitations of genetic fingerprinting?

• Contamination: A crime-scene sample can be contaminated with DNA from other individuals (first responders, laboratory staff), producing a mixed profile that is hard to interpret.
• Identical twins: Because identical twins arise from the same fertilised egg, they share exactly the same DNA profile. Genetic fingerprinting cannot distinguish between them.
• Database bias: The chance of a "cold hit" matching an innocent person increases when very large databases are searched, particularly if the suspect is from an ethnic group under-represented in the database.
• Degraded samples: Very old or heat-damaged DNA may produce incomplete profiles, reducing statistical certainty.

Frequently asked questions

Q: What is the difference between genetic fingerprinting and DNA sequencing?

Genetic fingerprinting analyses only a small number of specific repeat regions — it does not read the entire genome. DNA sequencing determines the exact order of every base pair in a DNA strand or gene. Fingerprinting is faster and cheaper, which is why it is used for identification; sequencing is used when the complete genetic information is needed, for example to diagnose a point mutation.

Q: Why do children share some bands with each parent?

Each person inherits one copy of every chromosome from each biological parent, so half of a child's STR alleles come from the mother and half from the father. On an autoradiograph, roughly half the child's bands should match the mother's profile and the other half should match the father's. If a band in the child cannot be matched to either the mother or the alleged father, paternity can be excluded.

Q: How does PCR improve genetic fingerprinting?

The polymerase chain reaction amplifies tiny amounts of DNA exponentially — even a single cell can yield enough material for analysis. This makes STR profiling possible from extremely small or degraded samples such as a single hair root, a touch DNA trace on a surface, or ancient bone material. Without PCR, traditional VNTR analysis required a relatively large, intact DNA sample.

Q: Can genetic fingerprinting be used to find out about a person's health?

Standard forensic genetic fingerprinting profiles only the STR regions used for identification — these are in non-coding DNA and do not reveal medical information. However, if a full genomic sequence is obtained (a different and more expensive process), it can reveal predispositions to certain conditions. Laws in many countries, including the UK, restrict how DNA profiles held on national databases may be used.

Q: What does it mean if two profiles "match"?

A match means the banding patterns at all tested loci are identical. This does not mean the samples are definitely from the same person — it means the probability of them being from two different unrelated individuals is extremely low (often stated as a random match probability). Courts consider this statistical evidence alongside other forensic and circumstantial evidence rather than treating a match as absolute proof.

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