DNA Evidence: A POWERFUL TOOL

By Hanson, Doug | Law & Order, April 2007 | Go to article overview
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DNA Evidence: A POWERFUL TOOL


Hanson, Doug, Law & Order


You are working the murder rape case of a woman in her early 30s. There is no semen at the scene, but this time, you think you got lucky. The CSI techs have found a lot of tissue under the woman's fingernails. She must have put up a good fight. There should be a lot of scratches on the perp, and better yet, there should be a lot of his DNA in those cells. The DNA lab's report is due any minute now.

The phone rings. It's the lab, "Sorry there is no match with the perp's DNA in CODIS." So here you are back at square one. There were three men observed in the area about the time of the crime but no eye witnesses. None of these guys has any record of violence or criminal activity. So now what do you do?

Suppose that you could ask the lab to run a special series of tests on this sample. The tests would give you a profile of the perpetrator and a reconstruction of this person from the information stored in his (or her) DNA: A profile of his height, weight, hair color, eye color, skin pigmentation, and other characteristics, or maybe even an indication of his facial morphology. Then this data could be fed into a computer system that could generate a 3-D image of his face and body. No, this is not something from the SciFI channel; it is what the future of DNA analysis could potentially bring, and it may be only five to 10 years away.

The DNA Database

Every person carries around his own DNA database. A database of all the physical characteristics of each one of us is stored in the four-letter code that makes up our DNA. It is like each of us at birth has a built-in parts list, showing the specifications of each part of our body. All we need to do is know how to read this parts list in order to reconstruct a profile of one's face, eye and hair coloration, body shape, etc. Once we know how to read the DNA information, then all we need to do is to find which genes program for blue eye color and which ones determines brown eye color. After that, we determine the genes for hair color, then ancestry, size of the ears, shape of the eyes, etc.

While all this sounds relatively straight forward, unraveling the information encoded in the DNA molecule has taken a tremendous amount of scientific research over the past 54 years since the structure of DNA was actually determined (1953). It is estimated that there are about 25,000 genes encoded in the human DNA molecule. Complicating matters further is the fact that each one of us gets half of our genetic information from our mother and the other half from our father. So each person's genetic makeup is a composite of the genes of both of our parents. That makes the job of interpreting results a little more complicated.

Forensic DNA analysis began in the mid 1980s. From current DNA analysis, we can tell if we have a match to a person in CODIS (the FBI's Combined DNA Index System), in the database subset of convicted offenders, or from a suspect who has provided a DNA sample. A variety of techniques are used in forensic DNA analysis. However, CODIS data is based on the analysis of specific regions or loci within the nuclear DNA molecule. These regions called STR, or short tandem repeat, segments have been found to be accurate predictors of a person's identity. CODIS uses a standard set of 13 STR regions for the basis of its identification index. The probability of two people having a match at all 13 locations in the DNA is in the order of one in one billion. For forensic identification, 10 of the 13 loci must be matched. For the sexual offender cases, all 13 loci must match in the DNA sample.

Usually, it is possible to tell the gender of the person from the DNA sample. Finding markers on the Y chromosome in a DNA sample indicates that the sample is from a male because the Y chromosome is passed from the father to the son and is not found in female samples. A vast amount of research is currently being carried out in an attempt to refine forensic DNA techniques, develop new assays and determine more specific DNA loci that can be used for identification purposes.

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DNA Evidence: A POWERFUL TOOL
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