3.8.4.3 Genetic fingerprinting

Genetic fingerprinting is a diagnostic tool used widely in forensic science, plant and animal breeding, and medical diagnosis. It is based on the fact that everyones DNA is difference (except identical twins). It relies on the fact that the genome of most eukaryotic organisms contains many repetitive non-coding bases of DNA. DNA bases which are non-coding are known as VNTRs. For everyone the number and length of VNTRs has a unique pattern. The probability of two individuals having the same VNTRs is very low.

Gel electrophoresis
This is used to separate DNA fragments according to their size. The fragments are placed on an agar gel and a voltage is applied across it. The resistance of the gel means that the larger the fragments the fess they move. In this way fragments of different lengths are separated. If fragments are labelled (e.g with radioactive DNA probes) their position in the gel can be determined by placing a sheet of X-ray film over the agar gel for several hours. The radioactivity from each DNA fragment exposes the film and shows where the fragment is situated.


Genetic fingerprinting
This consists of 5 main stages:

• Extraction
• extract the DNA from the sample by separating it from the rest of the cell
• Digestion
• cut the DNA into fragments
• Separation
• separate the fragments by gel electrophoresis under the influence of electrical voltage
• the gel is immersed in alkali to separate the double strands into single strands
• Hybridisation
• DNA probes are used to bind with VNTRs. Different probes for different target DNA sequences
• Development
• X-ray film is put over the nylon membrane

The pattern of bars of each sample is passed through an automated scanning machine which calculates the length of DNA fragments.

Genetic relationships and variability

Genetic fingerprinting can help resolve questions of paternity. It is also useful in determining genetic variability within a population.


Forensic science
Genetic fingerprinting can establish whether a person is likely to have been at the scene of a crime.

Medical diagnosis
Genetic fingerprinting can help diagnose diseases such as Huntington's disease.


Plant and animal breeding
Genetic fingerprinting can be used to prevent undesirable inbreeding during breeding season. It can also identify plants/animals that have a particular allele of a desirable gene.