DNA profiling is a forensic method used to identify individuals by characteristics of their DNA. A DNA profile is nothing but a set of DNA variations that is very likely to be different in all unrelated individuals, thereby being as unique to individuals as are fingerprints.
Developed in 1985, DNA profiling is used in, for example, parentage testing, criminal investigation and to identify a person or to place a person at the point of occurrence of any crime. It is now employed globally in forensic science to facilitate the law enforcing authorities.
The process, developed by Professor Sir Alec Jeffreys, begins with an individual’s DNA sample. The most desirable method of collecting a reference sample is the use of a Buccal swab, as the possibility of contamination remains to be minimal in the case.
In case this is not available, other methods may be used to collect a sample of blood, saliva, semen, or other appropriate fluid or tissue from personal items or from stored samples. Samples obtained from blood relatives can provide an indication of an individual’s profile, as could human remains that had been previously profiled.
A reference sample is then analyzed to create the individual’s DNA profile for forensic DNA testing. The DNA profile is then compared against another sample to determine whether there is a genetic match or not.
DNA, in this, is collected from cells, such as a blood sample, and cut into small pieces using a restriction enzyme, generating thousands of DNA fragments of differing sizes as a consequence of variations between DNA sequences of different individuals. The fragments are then separated on basis of size using gel electrophoresis.
Separated fragments are then transferred to a nitrocellulose or nylon filter; this procedure is called a Southern blot. The DNA fragments within the blot are permanently fixed to the filter, and the DNA strands are denatured.
Radiolabeled probe molecules are then added that are complementary to sequences in the genome containing repeat sequences. These repeat sequences tend to vary in length among different individuals and are called variable number tandem repeat sequences or VNTRs.
Probe molecules hybridize DNA fragments containing the repeat sequences and excess probe molecules are washed away. The blot is then exposed to an X-ray film. Fragments of DNA that have bound to the probe molecules appear as dark bands on the film.
The risk of contaminated-matching, in practice, is much greater than matching a distant relative, such as contamination of a sample from nearby objects, or from left-over cells transferred from a prior test.
The risk is greater for matching the most common person in the samples: Everything collected from, or in contact with, a victim is a major source of contamination for any other samples brought into a lab. For that reason, multiple control-samples are typically tested in order to ensure that they stayed clean, when prepared during the same period as the actual test samples.
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