Chapter Overview: Genetic Fingerprinting
Welcome to your study guide for Genetic Fingerprinting! This topic is part of your CCEA A2 5 unit: Genetics, Stem Cell Research and Cloning. Genetic fingerprinting (also known as DNA profiling) is one of the most powerful tools in modern biology. Whether it is solving a crime, confirming a biological parent, or saving endangered species from extinction, this technique allows scientists to identify individuals using their unique genetic code.
Don't worry if the laboratory steps seem complex at first. We will break down the biological theory, walk through each stage of the process step by step, and examine real-world applications so you feel completely confident for your exam.
1. The Biological Basis of Genetic Fingerprinting
To understand how genetic fingerprinting works, we first need to look at what makes your DNA unique.
Coding vs. Non-Coding DNA
About \(99.9\%\) of human DNA is identical between individuals. The genes that code for functional proteins (such as haemoglobin or insulin) are virtually the same in all of us. However, the majority of the human genome consists of non-coding DNA—sections that do not code for proteins.
Hypervariable Regions: VNTRs and STRs
Within this non-coding DNA lie special repeating sequences called hypervariable regions. These include:
• VNTRs (Variable Number Tandem Repeats): Longer sequences of DNA bases that repeat back-to-back multiple times.
• STRs (Short Tandem Repeats): Shorter sequences of repeating DNA bases (typically 2 to 6 base pairs long).
Why is this useful? The exact sequence of bases within a repeat is the same, but the number of times the sequence repeats varies enormously from person to person. For example, at a specific locus (position on a chromosome), you might have a sequence repeated 12 times, while your friend has it repeated 28 times.
Because humans have two copies of each chromosome (one from each parent), an individual can have two different repeat lengths at each locus. When scientists examine multiple repeat loci across the genome, the resulting combination is completely unique to that individual—creating a genetic fingerprint.
Did you know? The only individuals who share an identical genetic fingerprint are monozygotic (identical) twins, because they develop from a single fertilized egg that splits into two embryos.
Key Takeaway: Genetic fingerprinting relies on differences in the number of tandem repeats (VNTRs and STRs) found in non-coding DNA. Except for identical twins, every person's pattern of repeat lengths is unique.
2. The Step-by-Step Laboratory Process
Examiners often ask you to describe the full experimental procedure for producing a DNA profile. A handy way to remember the sequence is the mnemonic: Every Athlete Drinks Gel Shakes Vigorously (Extraction, Amplification, Digestion, Gel electrophoresis, Southern blotting, Visualisation).
Step 1: Extraction (Isolation)
DNA must first be extracted from a biological sample. Common sources include:
• Blood
• Cheek cells (swabs)
• Saliva
• Semen
• Hair roots
The cells are broken open (lysed) and the DNA is chemically separated and purified from other cellular components like proteins and lipids.
Step 2: Amplification via PCR (Polymerase Chain Reaction)
In real life, crime scenes or historical remains often provide only tiny, microscopic traces of DNA, or the sample might be partially degraded. Scientists use the Polymerase Chain Reaction (PCR) to make millions of identical copies of the specific DNA target regions in a matter of hours.
Step 3: Digestion (Fragmentation)
The extracted DNA is cut into smaller pieces using specialised enzymes called restriction endonucleases (restriction enzymes).
• These enzymes recognize and cut at specific palindromic recognition sequences.
• Crucially, they cut the DNA outside the tandem repeat regions.
• Because people have different numbers of repeats, this cutting produces DNA fragments of differing lengths (sizes) between individuals.
Step 4: Gel Electrophoresis (Separation)
This is the central separation technique. The DNA fragments are separated according to their physical size:
1. The DNA fragments are loaded into small wells at one end of an agarose or polyacrylamide gel, which is submerged in an aqueous buffer solution.
2. An electrical current (potential difference) is applied across the gel, creating a negative cathode near the wells and a positive anode at the far end.
3. Direction of movement: DNA fragments carry a permanent net negative charge due to the presence of phosphate groups in their sugar-phosphate backbone. Therefore, all DNA fragments migrate away from the negative end and move toward the positive anode.
4. Separation by size: The gel acts like a microscopic mesh or obstacle course. Smaller (shorter) DNA fragments encounter less resistance from the gel matrix and move faster and further through the gel. Larger (longer) fragments move much more slowly and remain closer to the wells.
Analogy: Imagine a dense forest. A small child can dart through the trees easily and travel a long distance quickly, whereas a tall adult wearing a large backpack will get caught on branches and move much more slowly!
Step 5: Southern Blotting & Hybridisation
The agarose gel is delicate and the DNA fragments within it will gradually diffuse. To preserve the pattern:
1. The double-stranded DNA fragments are chemically separated into single strands.
2. The DNA fragments are transferred ("blotted") directly from the gel onto a sturdy nylon membrane.
3. Hybridisation: Single-stranded DNA probes are added. A probe is a short length of single-stranded DNA with a base sequence that is complementary to the specific VNTR or STR target sequences.
4. These probes are labelled with either a radioactive isotope (e.g., \(^{32}\text{P}\)) or a fluorescent marker.
Step 6: Visualisation (Development)
The unbound probes are washed away, and the final banding pattern is revealed:
• Radioactive probes: The nylon membrane is placed against X-ray film. The radioactivity exposes the film (a process called autoradiography), producing a series of visible dark bands.
• Fluorescent probes: The membrane is illuminated under ultraviolet (UV) light, causing the labelled bands to glow.
Key Takeaway: DNA fragments are negatively charged and migrate toward the positive anode during gel electrophoresis. Shorter fragments travel further and faster. Labelled probes hybridise to the fragments to make the banding pattern visible.
3. Applications of Genetic Fingerprinting
A. Forensic Analysis
Forensic scientists compare DNA profiles obtained from crime scene evidence (e.g., blood on a weapon) with profiles from suspects or DNA databases.
• Threshold for identity: For a suspect to be identified as the source of a crime scene sample, there must be a \(100\%\) complete match across every single band in the profile.
• Possible errors: A mismatched band or false positive/negative can occur due to cross-contamination of samples, degradation of old DNA, or errors during PCR amplification.
B. Paternity and Maternity Testing
A child receives exactly \(50\%\) of their nuclear DNA from their biological mother and \(50\%\) from their biological father. Consequently, every single band in a child's genetic fingerprint must be present in either the mother's or the father's profile.
How to solve paternity questions in exams:
1. Look at the child's profile band by band.
2. Match and eliminate all the bands that the child shares with the mother (the maternal bands).
3. Look at all the remaining bands in the child's profile (the paternal bands).
4. The true biological father must possess all of these remaining bands. If an alleged father is missing even one of these non-maternal bands, he cannot be the biological father.
C. Medical Screening and Diagnostics
Genetic fingerprinting can detect specific alleles associated with inherited disorders (such as Huntington's disease or cystic fibrosis) or identify healthy individuals who are carriers of a recessive disease allele.
D. Conservation Biology and Animal Breeding
In captive breeding programmes (such as those in zoos), genetic profiles are used to assess the genetic diversity of a population. By identifying how closely related individuals are, conservationists can prevent inbreeding, maintain genetic variation, and increase the health and survival rate of endangered species.
Key Takeaway: In forensics, a \(100\%\) match is required. In paternity testing, any band in a child's profile that does not come from the mother must be present in the biological father's profile.
4. Common Exam Pitfalls & Tips
Pitfall 1: Confusing size with electrical charge
Incorrect: "Smaller fragments move further because they have a stronger negative charge."
Correct: All DNA fragments have a negative charge and move toward the positive anode. They separate strictly because smaller/shorter fragments pass through the pores of the gel matrix more easily than larger fragments.
Pitfall 2: Expecting a child to match all of the father's bands
Incorrect: "Father 1 is the dad because his bands look just like the child's."
Correct: A child only inherits half of each parent's genetic material. The child will not show all of the father's bands; instead, all of the child's unmatched bands must be present in the father.
Pitfall 3: Forgetting key terminology
Always explicitly name the technique gel electrophoresis when describing how fragments are separated, and name restriction endonucleases as the enzymes that cut DNA.
Quick Summary Checklist
• Basis: Non-coding VNTRs/STRs vary in repeat length between individuals (except identical twins).
• 6 Steps: Extraction \(\rightarrow\) PCR Amplification \(\rightarrow\) Digestion (Restriction Endonucleases) \(\rightarrow\) Gel Electrophoresis \(\rightarrow\) Southern Blotting \(\rightarrow\) Visualisation (X-ray / UV).
• Electrophoresis Principle: DNA is negatively charged (phosphate backbone) \(\rightarrow\) moves to positive anode \(\rightarrow\) smaller fragments travel further.
• Probes: Single-stranded complementary sequences with radioactive or fluorescent labels.
• Uses: Forensics (\(100\%\) match), Paternity (\(50\%\) maternal / \(50\%\) paternal), Medical screening, Conservation breeding.