There are a lot of myths surrounding DNA evidence. For people who view crime related television shows, it seems as though DNA is used in every case, and the DNA results can be obtained in a few hours or a day at most. It also appears that DNA evidence is absolute, always convicting or exonerating the suspect. In real life, DNA testing is a much slower and more cumbersome process, whose accuracy depends on the procedures used and the reliability of the people doing the testing.
DNA stands for Deoxyribonucleic Acid, which is a substance found in every cell of every living thing on earth. The DNA strands in each cell contain the entire chemical code to reproduce an entire person. Somewhere in the strands of the DNA are codes for such things as hair color and texture, eye color, the shape of a person’s face, and so forth.
Chemical DNA testing does not focus on the entire strand of DNA. Rather, the chemical test focuses on a section of the DNA called the STR (Short Tandem Repeat) section that varies widely from person to person, which makes it easier to test.
Anyone who has taken high school biology knows what DNA looks like: a double helix connected by cross bars, rather like a ladder that has been twisted over and over again. Each crossbar represents a locus, and on each locus is an allele – a single genetic location which codes for a single genetic trait – such as blue eyes, for example.
DNA testing does not test for what the alleles actually do. It is a comparative test only, and it tests for a a particular allele on each locus.
The more loci are tested, the more accurate the test. As of this writing, the prosecution in Los Angeles County typically tests for eighteen or nineteen loci.
Typically, the prosecution will have a DNA sample taken from a crime scene, and then they will have a reference sample taken from an individual, and the lab will be asked to establish whether the crime scene sample matches the reference sample.
Defenses against a DNA test typically center on the process used in collecting the crime scene sample, and the chain of custody when it is taken to the lab, as well as the lab’s testing procedures. Contaminated DNA can yield ambiguous or erroneous results, particularly where a DNA sample contains DNA from more than one individual.
If you have a case involving DNA evidence, you need a defense attorney who has experience and expertise in handling DNA cases.
Photo credit: By David H. Lee, MD, FRCPC; Penny A. Henderson, ART; Morris A. Blajchman, MD, FRCPC [CC-BY-SA-2.5 (www.creativecommons.org/licenses/by-sa/2.5) or CC-BY-SA-2.5 (www.creativecommons.org/licenses/by-sa/2.5)], via Wikimedia Commons