DNA fingerprinting distinguishes one individual from another by analyzing specific regions of their DNA that exhibit high variability among individuals. These regions, often referred to as genetic markers, contain repetitive DNA sequences known as short tandem repeats (STRs). Here's how DNA fingerprinting differentiates individuals:
- STR Loci Selection: DNA profiling targets multiple STR loci for analysis. These loci consist of specific DNA sequences with varying numbers of repeats. The number of repeats at each locus can differ significantly between individuals, resulting in unique combinations of alleles.
- Amplification of DNA: The DNA sample is amplified using the polymerase chain reaction (PCR). Primers specific to the flanking regions of the targeted STR loci are used to selectively amplify those regions. PCR exponentially amplifies the DNA, generating millions of copies of the target regions.
- Fragment Analysis: The amplified DNA fragments are separated based on size using gel electrophoresis or capillary electrophoresis. The fragments are loaded onto a gel or capillary, and an electric current is applied to facilitate their migration. The smaller fragments move faster and migrate further through the gel or capillary, resulting in distinct bands or peaks.
- Banding Pattern Visualization: The separated DNA fragments are visualized by staining the gel or using fluorescent labels. The resulting banding pattern or electropherogram shows the sizes of the DNA fragments. Each band or peak corresponds to a specific allele at a particular STR locus.
- DNA Profile Comparison: The DNA profiles of different individuals are compared by analyzing the banding patterns or peak heights. If the number and sizes of the DNA fragments at multiple STR loci match between two individuals, it indicates a high probability of a genetic relationship or, in the case of forensic analysis, the presence of a match between crime scene evidence and a suspect's DNA.
- Statistical Analysis: The probability of two individuals having the same DNA profile is calculated using population genetics statistics. This probability, often referred to as the match probability or random match probability, helps assess the significance of a DNA match and the rarity of the profile within the population.
By analyzing multiple STR loci and their alleles, DNA fingerprinting generates a unique profile for each individual. The chance of two unrelated individuals having identical DNA profiles at multiple STR loci is exceedingly low, making DNA fingerprinting a highly effective tool for individual identification and distinguishing one person from another.