What is genetic fingerprinting?

Genetic fingerprinting, sometimes called DNA profiling, is a technique used to identify unique DNA patterns in individuals, to help identify individuals in forensics or family relationships.

Variable number tandem repeats and short tandem repeats

Genetic fingerprinting relies on the fact that, with the exception of identical twins, every person's DNA sequence is distinct. This is due to variation in the sequence and length of unique non-coding, repetitive DNA segments called variable number tandem repeats (VNTRs, sometimes called minisatellites).

Key features of VNTRs:

• They are present across the genomes of most eukaryotes.
• They are not involved in protein coding.
• They have extensive variability in sequence and length among individuals.
• Their length and location are heritable.
• The chance of two individuals who are not identical twins sharing identical VNTR patterns is very low.
• A high similarity in VNTR patterns indicates two individuals may be closely related.

Short tandem repeats (STRs, sometimes called microsatellites) are repeated sequences of nucleotides that are smaller than VNTRs and can also be used in genetic fingerprinting.

The process of creating a genetic fingerprint

Creating a genetic fingerprint or DNA profile involves five main steps.

These are as follows:

1. DNA extraction - DNA is extracted from a tissue sample and amplified using PCR.
2. DNA digestion - Restriction enzymes are used to cut the DNA into fragments at points near the VNTR sequences.
3. Fragment separation - Gel electrophoresis separates the fragments by size, and they are denatured to produce single strands.
4. Hybridisation - Specific radioactive or fluorescent probes bind to complementary VNTR sequences.
5. Development - The positions of the probes are revealed, creating a barcode-like pattern of DNA bands unique to each individual.

If radioactive probes are used, X-ray images are taken of the paper or membrane during the development stage. If fluorescent probes are used, the paper or membrane is placed under UV light causing the fluorescent probes to glow.

The probability of a match can then be determined by computer analysis of the band patterns. The higher the number of matching bands, the greater the likelihood that the fingerprints are from the same individual.

Applications of genetic fingerprinting

Genetic fingerprinting is a vital tool in determining genetic identity and in various fields such as medicine, agriculture, and forensics.

Key uses of genetic fingerprinting technology:

• Establishing paternity by matching bands on a child's genetic fingerprint to those of potential parents and assessing the similarities between VNTR patterns.
• Identifying suspects from crime scene DNA (e.g. in blood, semen, saliva, skin cells, or hair roots).
• Supporting criminal convictions with match probability calculations.
• Identifying the risk of genetic disorders and predicting their onset and severity.
• Selecting desirable traits in plants and animals for selective breeding while preventing severe inbreeding.
• Evaluating genetic diversity by comparing the variety of genetic fingerprints within a population.

Limitations of genetic fingerprinting

There are a few limitations of genetic fingerprinting.

These limitations include:

• Environmental contamination may compromise results.
• Close genetic relatives could have similar fingerprints.
• Assumptions about genetic variation in populations underpin probability calculations, meaning genetic fingerprinting does not prove guilt or causation and other evidence must also be taken into account.

Despite these limitations, genetic fingerprinting remains an invaluable tool, with its accuracy continually enhanced as DNA sequencing technologies advance.