Q. What is genetic fingerprinting? How valuable is it in crime investigations and tracing one's parentage?
A. Genetic fingerprinting (DNA testing; DNA typing; DNA profiling) refers to the process of identifying an individual by studying his DNA. DNA is short for deoxyribose nucleic acid, a molecule that can be found in the nucleus of each and every cell of the body, except for the red blood cells which do not have a nucleus.
In the nucleus, the DNA molecules are contained in structures called chromosomes, of which there are 23 pairs in humans. Half of the 23 pairs of chromosomes that a person has have been inherited from his/her mother and the other half from his/her father.
The DNA molecule in each chromosome is in the form of a very long but thin thread that consists of two twisted strands made up of sugar and phosphate molecules. At regular intervals, nitrogen-containing bases connect the two strands to each other-much like the two-sides of a ladder being connected by rungs.
The human genome, which refers to the total amount of DNA present in the 23 pairs of chromosomes, contains three billion nitrogen bases. The sequencing or arrangement of these bases (DNA sequence) in the DNA strands within the chromosomes is unique to a person. The probability that two living persons have identical DNA sequence (except for identical twins) is practically zero. The complicated process of development that the egg cell (of a person's mother) and the sperm cell (of a person's father) undergo before these two cells unite during fertilization to produce the fetus that will eventually become the person ensures this.
Genetic fingerprinting involves obtaining segments of a person's DNA from certain chromosomes and comparing the sequence of the bases in these segments with those of another person or particular population or family groups and identifying DNA sequences, called genetic markers, which the person have in common with the others. Statistical analysis of this comparison can help resolve questions regarding a person's ancestry and help forensic investigators solve crimes. Incidentally, for samples, genetic fingerprinting utilizes nucleated body cells, which can come from any body tissue such as a single root of hair, saliva, urine or semen.
Genetic fingerprinting is a more accurate method of identification than fingerprinting, blood typing or any other procedure that is presently being employed. But in its current state of development, genetic fingerprinting has limited applications. In forensic medicine, it is useful in acquitting innocent suspects, but not in convicting guilty ones. The test cannot tell whether the DNA sample taken from the crime scene comes from the suspect, but it can tell with certainty if it does not come from the suspect.
Similarly, in verifying the parentage of a child, genetic fingerprinting cannot definitely say whether the person who is undergoing the test is the mother or father of a child, but it can tell with certainty if he or she is not the parent.
Genetic fingerprinting also has other advantages over other identification procedures. Because DNA is resistant to degradation, genetic fingerprinting can still be done even if the sample has been treated with detergents, acids, bases, oil-based products, salt, and even if it has been contaminated by bacteria. Additionally, DNA degrades very slowly hence, samples that are already several years old or those taken from corpses that have been buried for years can still be used.
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The world of genetic fingerprinting