DNA profiling harnesses the natural genetic variation found in specific regions of an individual's DNA to create a unique genetic fingerprint, enabling differentiation between people.
Understanding Genetic Variation in DNA Profiling
The foundation of DNA profiling lies in the fact that while much of our DNA is identical, certain segments exhibit considerable variability from person to person. This genetic variation is the key to individual identification.
The Role of Short Tandem Repeats (STRs)
Traditionally, DNA profiling (also known as DNA fingerprinting or genotyping) primarily utilizes short tandem repeats (STRs). These are specific sequences of DNA where a short pattern of nucleotides (e.g., GAAT) is repeated multiple times in a row.
The crucial aspect of STRs is highlighted by the fact that:
"The number of times a sequence repeats varies from person to person, and this variation can be used to tell people apart."
For example, one person might have the sequence 'GATAGATA' repeated 10 times at a particular STR locus, while another person might have it repeated 15 times at the exact same locus. This difference in the number of repeats is the genetic variation that DNA profiling exploits.
The Process of Creating a DNA Profile
Creating a DNA profile involves several key steps that capitalize on these STR variations:
- DNA Extraction: DNA is first isolated from a biological sample (e.g., blood, saliva, hair, skin cells).
- Amplification (PCR): Specific STR regions are then amplified using a technique called Polymerase Chain Reaction (PCR). This creates millions of copies of these specific DNA segments, making them detectable.
- Separation and Detection: The amplified STR fragments are then separated by size, typically using a method like capillary electrophoresis. Shorter fragments (fewer repeats) travel faster than longer fragments (more repeats).
- Profile Generation: The data from different STR loci are analyzed. Each person inherits two copies of each STR locus (one from each parent), and the combination of the number of repeats at multiple designated STR loci creates a highly distinctive numerical profile.
Example of STR Variation
Consider two hypothetical individuals and their STR profiles at just two different loci:
| STR Locus | Individual A (Repeats) | Individual B (Repeats) |
|---|---|---|
| D3S1358 | 15, 17 | 14, 16 |
| vWA | 16, 18 | 16, 17 |
As you can see, even with only two loci, the specific combination of repeat numbers for Individual A (15,17; 16,18) is different from Individual B (14,16; 16,17). When this analysis is performed across 15-20 or more standard STR loci, the probability of two unrelated individuals sharing the exact same profile becomes astronomically low, making it a powerful tool for identification.
Practical Applications of DNA Profiling
The ability of DNA profiling to distinguish individuals based on genetic variation has revolutionized various fields:
- Forensic Investigations: A primary application is in forensic science, where DNA samples from crime scenes (e.g., blood, semen, hair) can be matched to suspects or used to identify victims. This provides strong evidence that can link individuals to criminal acts or exonerate the wrongly accused.
- Paternity and Kinship Testing: DNA profiling is widely used to establish biological relationships, such as paternity or maternity, by comparing the STR profiles of a child and potential parents.
- Disaster Victim Identification (DVI): In mass casualty incidents, DNA profiling helps identify victims, even when traditional methods are impossible due to severe fragmentation or decomposition of remains.
- Missing Persons Cases: DNA from unidentified remains can be compared against DNA profiles from relatives of missing persons databases.
By focusing on the varying number of repeats within STR sequences, DNA profiling effectively leverages natural genetic differences to create highly individualistic genetic fingerprints.
