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This is the year of medical mysteries, if you watch the new CSI spin-offs dotting the fall TV line-up. You will notice that much of the plot material will utilize DNA analysis. It has become the cornerstone of death analysis. It has helped grieving families identify lost loved ones, aided in resolving paternity suits, rape cases and violent crimes. DNA analysis has brought forensics out of the dark ages. It is used everywhere now, but in Ontario there are only two forensic centres that can perform it.
The whole process relies on the fact that every cell in your body has the same blueprint or DNA. And your DNA is made up of pairs of chromosones which are derived from your parents on an equal-share basis. Hair cells, blood cells and semen cells all have identical fingerprints. To start a DNA analysis, you need to start with a small bit of material that is essentially pure and uncontaminated. The amount actually required is microscopic. One-billionth of one gram of material will contain hundreds of cells to produce a DNA profile. Solvents are added to the cellular material to extract the DNA, and then the sample is placed into the analyzer which employs an electric field to sort the biochemical bits into piles which appear as bands. The bands correspond to genetic loci. These days, at least 9 different loci are used to characterize the sample. The process then involves comparing the sample to another sample of interest. What results is a probability that the two samples could have come from the same person. On TV, this is called a positive match. What it really means is that there is a 1 in 10 billion chance that both samples did not come from the same person. With only 7 billion people on the planet, it seems that the present criteria would suffice. The exception is identical twins who of course have identical DNA.
Contamination is still the Achilles heel of the process. It simply obscures the ability to compare. For example, consider a sample taken from the vagina. It may contain genetic and non-genetic material from the sample donor. It may also contain semen and sperm cells. One may argue that sperm is a gamete with only half the genetic material, the sample will contain hundreds of cells which will be disrupted and clumped together. This will reveal the full genetic fingerprint. In such a case, the DNA analysis may produce a complex pattern. However, a second pure donor sample can be taken from the donor female and used to provide a pure DNA analysis. This second analysis can be subtracted from the more complex pattern. What is left is an identifying genetic fingerprint. The result must still be compared to a sample obtained from a suspect to be effective. And this is another short-fall of the process. The analysis cannot identify a culprit at random. In this regard ethical arguments abound about DNA libraries and registries. Old crime cases have been solved when an old profile suddenly matches the sample provided by a suspect in a new investigation. This makes it an excellent and acceptable tool.
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