If Sherlock Holmes were working today, he would be adept at genetic fingerprinting. This is a very powerful tool in forensic science. It is equally powerful in its ability to identify or eliminate suspects.
Some people in fact are so impressed they propose the establishment of a national data bank of DNA samples from every citizen to check out those suspected of major criminal activity. This idea is premature.
DNA is the chemical that comprises our genetic information. Our bodies are composed of cells and the adult body contains about 50 trillion cells. With the exception of sex cells (sperm and egg) and red blood cells, every cell contains a copy of our entire genetic blueprint.
Half your genetic material comes from your mother, and half from your father: two sets, each covering the same ground.
Your sex cells contain only one set, but, when the sperm combines with the egg, the normal two sets of genetic material per cell is restored.
DNA is a very long molecule composed of two strands wound about each other in a double helix. Each strand is made from a linear sequence of units called nucleotides.
There are four types of nucleotide in DNA, denoted by the letter symbols A, T, G, C. This is the letter alphabet of the DNA language.
There are only 64 words in this language, and each word contains three letters.
DNA contains the genetic information but it acts through the agency of proteins.
Proteins constitute most of the dry matter of the body. Proteins have an important structural role, but, more importantly, all enzymes are proteins.
Life is sustained by a complicated orchestra of thousands of inter-related chemical reactions (metabolism).
These reactions must proceed quickly and accurately enough to sustain life and this is made possible by catalysts called enzymes. Catalysts speed up chemical reactions but are not themselves consumed in the reactions.
A protein is composed of amino acids joined in linear array. There are thousands of different types of protein but only 20 different amino acids.
The exact sequence of amino acids in a protein determines its nature or type. The information that specifies the sequence of amino acids in every protein in the cell resides in the three-letter code written in the DNA. The length of DNA sufficient to code for a single protein is called a gene.
The sequence of polynucleotide letters in your DNA is unique, unless you are an identical twin, in which case both have the same sequence. This unique sequence is the basis for DNA fingerprinting. For identification purposes it is sufficient to determine the sequence in part of your DNA.
I described how DNA codes for proteins do, but the great majority of your DNA doesn't, code for protein, and biologists are unsure as to its function. It is sometimes called "junk" DNA.
Located throughout this junk DNA are islands of repeat patterns of nucleotide letters. Unless you are an identical twin, the pattern of repeats in your DNA is unique and it is these repeat patterns that are detected in DNA fingerprinting.
How reliable is DNA fingerprinting? Three types of objection have been raised - (a) the possibility of human error; (b) the possibility of false positive results, and (c) experts disagree on the reliability of the method.
These objections are competently discussed, and dismissed, by Richard Dawkins in his latest book, Unweaving the Rainbow (Penguin, 1998).
Dawkins points out that mistakes can be made in handling any sort of evidence. This is not a factor unique to DNA fingerprinting.
Biological sample collection at a crime scene for DNA fingerprinting should be carried out according to strictly defined guidelines, as should the collection of any other evidence.
The fingerprint evidence can then speak for itself unblemished by doubts about sample contamination or identification.
What about the possibility of false positive evidence? In other words, a crime is committed with which I have no association but a DNA fingerprint from the scene of the crime happens by chance to be identical to my DNA fingerprint.
This is a question of probability. The chance that someone else's DNA fingerprint is identical to mine is extremely small, at worst thousands to one against, and, more likely, millions to one against. Experts disagree on the possibility of DNA fingerprinting yielding false positive results. But Dawkins points out that even the most pessimistic estimate of the probability of a false positive with DNA fingerprinting is still much better than the most pessimistic estimate of false positives arising with other types of evidence, e.g., eyewitness identification.
WHAT about the case for establishing a DNA data bank of all citizens against which suspects in serious crimes, such as murder, rape, etc, could be crosschecked?
If it could be guaranteed such a data bank would be used exclusively for this purpose, I would find it difficult to argue against it.
After all, such a system would threaten nobody but the criminal.
But the problem is how to guarantee the DNA data bank will not be used for other purposes.
It would be a gross violation of personal privacy if government or some other agency used the data bank to screen for genetic diseases, or genetic predisposition to various conditions.
Most people feel, regardless of promises, such a DNA data bank would enable some future unscrupulous operator to screen for purposes other than detection of criminal activity.
For the time being this fear is undoubtedly real enough to prevent the establishment of a DNA data bank.
William Reville is a senior lecturer in biochemistry and director of microscopy at UCC.