Admissibility of Dna in Criminal Cases: a Colorado Primer
| Publication year | 1995 |
| Pages | 2333 |
1995, October, Pg. 2333. Admissibility of DNA in Criminal Cases: A Colorado Primer
Deoxyribonucleic acid ("DNA") includes all of the instructions for the assembling and regulation of all life forms. DNA in humans is found in virtually every cell, including blood, semen, saliva and hair roots.
Chromosomes that carry DNA are rod-like structures that are actually a long double helix, which looks like a twisted ladder. A DNA molecule is 3.3 billion "rungs" long. More than 99 percent of these rungs are exactly the same from person to person. In humans, fewer than 1 percent of the rungs dispersed throughout the forty-six chromosomes are randomly variable between different individuals. Approximately 3 million sites vary from person to person. DNA technology makes it possible to study these variations directly.
Obviously, if a DNA profile examined all 3 million sites of variation, each person's DNA could be individualized. Such an endeavor is currently unduly burdensome in terms of time, labor and cost. As an alternative, forensic scientists have developed a process called RFLP analysis to isolate and compare the variable regions of DNA.
RFLP analysis essentially samples the variable portions of DNA by using radioactive DNA fragments, or "probes," that attach themselves to processed crime scene samples. These probes form a pattern that resembles a supermarket bar code. The finished product, which contains the pattern of both the crime scene sample and the suspect's sample, is called an autorad. The bands on the autorad are measured both by a visual method and by computer. A "match" will be declared if the pattern in the suspect's sample matches the sample obtained from the victim or the crime scene.
After determining that a match has occurred, there must be a determination as to how frequently the pattern produced by the DNA fragments or probes occurs within a given population because, unlike fingerprints, the science behind DNA analysis is not yet able to conclude that a particular pattern yielded from the probes is unique.
The statistical analysis accompanying the evidence of a match is accomplished by comparing the suspect's pattern to a randomly selected database compiled by the testing laboratory. Each person in the database has had his or her sample "probed" in the same way as the suspect's. The resulting patterns occurring on autorads are the database. Computations are then done to determine how frequently the pattern seen in a suspect is seen in the database.
This computation is done by calculating the frequency of each probe, then calculating the frequency of the genotype and lastly by determining the frequency of the suspect's entire pattern by multiplying the genotype frequencies for each probe, using a formula called the "product rule." The product rule can be applied as long as the events that are being multiplied together are independent events.(fn2)
PCR, which stands for "polymerase chain reaction," is also sometimes known as DNA amplification. The test produces a set of dots that indicate whether specific DNA characteristics are absent or present in a given sample. This test yields results that are less specific than those samples tested by RFLP analysis. The biggest advantage of PCR testing is that it requires far less biological material than RFLP analysis for DNA testing.
The earliest reported decisions in the United States involving the admissibility of forensic DNA evidence are dated 1988, about one year after the technique was used in a murder case in England.(fn3) Admissibility in DNA cases is and has been determined under either the Frye test (the general...
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