Genetic Fingerprinting and Forensics
Genetic fingerprinting (also called DNA profiling) is a technique used to identify individuals based on their unique DNA profile. No two individuals (except identical twins) have the same DNA fingerprint, making it an extremely powerful tool in forensic science, paternity testing, and other applications. This lesson covers the molecular basis of genetic fingerprinting, the techniques involved, and its applications.
The Molecular Basis of Genetic Fingerprinting
Variable Number Tandem Repeats (VNTRs) and Short Tandem Repeats (STRs)
Although approximately 99.9% of human DNA is identical between individuals, the remaining 0.1% contains highly variable regions that differ from person to person.
Much of this variation is found in repetitive DNA sequences — regions where a short sequence of bases is repeated multiple times in tandem (one after another).
Short Tandem Repeats (STRs)
Key Definition: Short tandem repeats (STRs) are sequences of 2–6 base pairs that are repeated a variable number of times at specific loci in the genome. The number of repeats at each locus varies between individuals.
- Also known as microsatellites.
- Example: The STR sequence "AGAT" might be repeated 8 times in one individual, 12 times in another, and 15 times in a third.
- STRs are the basis of modern DNA profiling systems. In the UK, the current system (DNA-17) analyses 17 STR loci plus a sex-determining marker (amelogenin).
- Because each person has two alleles at each locus (one from each parent), and the number of repeats at each locus varies, the combined profile across multiple loci is unique to each individual (probability of two unrelated people sharing the same profile is less than 1 in a billion).
Variable Number Tandem Repeats (VNTRs)
- VNTRs are longer repeat sequences (10–60 base pairs per repeat unit), also known as minisatellites.
- VNTRs were used in the original DNA fingerprinting technique developed by Sir Alec Jeffreys in 1984 at the University of Leicester.
- VNTRs have largely been replaced by STRs in modern forensic analysis because STRs are more amenable to PCR amplification and automated analysis.
The DNA Profiling Process (Using STRs)
Step 1: DNA Extraction
- DNA is extracted from the biological sample (blood, saliva, semen, hair root, skin cells, etc.).
- Even very small or degraded samples can yield a profile because PCR can amplify the DNA.
Step 2: PCR Amplification
- Specific STR loci are amplified using PCR with primers flanking each STR region.
- Modern systems use multiplex PCR, which amplifies multiple STR loci simultaneously in a single reaction (using different primer pairs for each locus).
- Primers are labelled with fluorescent dyes of different colours, allowing the products from different loci to be distinguished.
Step 3: Separation by Capillary Electrophoresis
- The amplified STR fragments are separated by capillary electrophoresis (a modern, high-resolution alternative to gel electrophoresis).
- Fragments are separated by size as they pass through a thin capillary filled with polymer gel.
- A laser detector reads the fluorescent labels and determines the size (number of repeats) and colour (which locus) of each fragment.
Step 4: Generating the DNA Profile
- The results are displayed as an electropherogram — a graph showing peaks at specific positions corresponding to the size of each STR allele at each locus.
- Each locus shows either one peak (if the individual is homozygous — same number of repeats on both chromosomes) or two peaks (if heterozygous — different numbers of repeats on each chromosome).
- The pattern of peaks across all loci constitutes the individual's DNA profile.
Southern Blotting (Original Method)
The original DNA fingerprinting method developed by Alec Jeffreys used Southern blotting to detect VNTRs. Although largely superseded by PCR-based methods, Southern blotting remains important for understanding the principles:
Procedure