CHAPTER 7 DNA
| Jurisdiction | United States |
Chapter 7 DNA
The ability of DNA profiling to identify the presence of a defendant at a crime scene has had an explosive effect on criminal trials. DNA typing is one of the newest forms of forensic identification and the most powerful. Its power stems from the wealth of knowledge scientists have generated in mapping the human genome — science for the sake of science, rather than for litigation. We can now examine a mere 13 sites on human chromosomes and, using probability statistics, identify a person with virtual certainty.
The scientific process by which DNA profiling has evolved has created pressure on other forms of forensic identification — such as fingerprints and handwriting — that rely on a scientific hypothesis, but require examiner subjectivity. What are the implications of DNA profiling for other forensic disciplines? Can any other discipline duplicate the power of DNA?
The phenomenon of reversals of convictions due to newly tested DNA is also a byproduct of DNA technology. Photos of convicts who have been cleared by DNA have caused the public to question the reliability of eyewitness identification, upon which many of the convictions have rested. Whether it will put pressure on juries to demand DNA evidence before they will convict is not yet known. However, the "CSI Effect," the desire of juries to want scientific evidence connecting a defendant with a crime, is probably here to stay.
Will DNA testing lead to fewer erroneous convictions and more confidence in the criminal justice system?
Although the National Research Council report in 2009 acknowledged that DNA was reliable science and did not need further testing, it called for improvements in accreditation, uniform standards across laboratories, and better training. It also noted the severe backlogs in many state laboratories, due in part to increased demand for DNA testing.
Finally, although the STR method of DNA testing and the PCR method of copying small samples of DNA are both now pretty much uniformly accepted as meeting the Daubert test, DNA continues to evolve new practices that result in objections from defendants. The latest development is what is called "Touch DNA" or Low Copy Number (LCN) DNA, which is lifted typically from skin cells off weapons and the like. The objection results because greater amplification is needed than the PCR process uses, which results in DNA profiles with what is called "stutter," peaks that may not be real. No doubt this method will ultimately receive court approval, but there will probably continue to be new DNA technologies developed in the future, as the ability of DNA to link suspects to crimes is powerful. Another such development is the Rapid DNA test kits designed to create a DNA profile in ninety minutes at a crime site or police station. No doubt results from these test kits will engender objections against admissibility.
Finally, the Confrontation Clause of the U.S. Constitution has led defendants to object if every person involved in the testing of DNA, from laboratory employee to supervisor, is not available for cross-examination. Although the U.S. Supreme Court in Williams v. Illinois,1 held in a plurality opinion that DNA test results were not "testimonial" and therefore did not trigger the Confrontation Clause, at least one court has restricted that opinion to its unusual facts and declined to follow it, holding that at least certain personnel involved in DNA testing must be present to testify. This issue can be expected to have widespread implications in the future.
Based on this chapter, students will be able to:
1. Explain the science of DNA testing and why testing of 13 loci can produce absolute identity.
2. Understand why DNA can absolutely rule out a suspect.
3. Describe the product rule.
4. Explain the prosecutor's fallacy.
5. Understand how the Confrontation Clause affects what witnesses must be available to testify to the DNA testing process.
6. Explain how conditions in DNA testing laboratories can affect DNA tests.
7. Evaluate the reasons why courts have "judicially noticed" the science of DNA testing.
8. Explain how to challenge DNA test results.
9. Understand the challenges to Low Copy Number or Touch DNA results.
10. Explain Polymerase Chain Reaction and why it is important to DNA testing.
11. Explain the difference between standard DNA tests and mitochondrial DNA testing.
12. Understand the objections to "Touch DNA" as reliable science.
What Is DNA?
The initials DNA stand for deoxyribonucleic acid, which is the material inside our chromosomes that controls our genetic makeup. All humans have 23 pairs of chromosomes — one pair inherited from the mother and one pair through the father. Both the egg and sperm contain only half the normal amount of chromosomes. The oocyte, or egg, contains 23 chromosomes and the sperm cell likewise contains 23 chromosomes. When the sperm fertilizes the egg, the cells merge and a full set of 46 chromosomes determines the genetic makeup of the offspring.
Chromosomes are made up of DNA, organized in a helix pattern, a ladder-like formation composed of sugars on each edge and four different proteins as the rungs of the ladder. These are adenine, thymine, guanine, and cytosine. Adenine pairs only with thymine and guanine pairs only with cytosine. They are organized in a pattern that repeats. These patterns vary by individual. By measuring where a pattern stops and begins a new repetition, scientists can distinguish one person's DNA from another.
At each genetic marker point on a chromosome, called a "locus," a person will have two genetic markers, called "alleles." One comes from the mother and one from the father. On the loci tested for DNA profiling, these alleles are of different lengths in different people, which is how a paternity or maternity test can identify offspring based on one set of shared chromosomes.
DNA Typing — A Quickly Changing Technology
DNA evidence has become the most respected forensic evidence in the shortest time of any forensic science. In 1944, Oswald Avery defined the concept of DNA (deoxyribonucleic acid) as the building block of human genetics. In 1953, Watson and Crick discovered the double helix model. In 1980, Botstein discovered small variations at the genetic level which he called restriction fragment length polymorphism. Shortened to its initials, it is RFLP. That technology was first applied in 1984 to detect alleles at various loci along a chain of DNA.
The Principles of DNA Testing
DNA testing does not create a profile of the total DNA of an individual. All humans share 99.9% of their DNA in common. Of the .1% of DNA that varies, there are approximately 2,000,000 markers (called loci for the location on the chromosome) that differ from person to person. Some loci control individual characteristics, such as eye color, height, or bone structure. Of these 2,000,000, there are about 2,000 markers that do not appear to control any known genetic variable. These are called "junk DNA" because their use is unknown. These junk DNA sites are used for testing because, as far as scientists know, they are distributed randomly through the population. Of these loci (sites), only 13 are tested in the typical DNA kit. How can a mere 13 markers out of 2,000,000 identify a person to a certainty of 1 in 300 million? The answer lies in statistics, which is another complexity involved in explaining DNA tests to a jury.
DNA testing can absolutely rule out or exclude a suspect. In the case of rapes, hospitals take samples of fluids from the vagina for testing in what is called a rape kit. If the DNA found in a rape kit does not match the suspect's, he is absolutely excluded as the rapist, or at least as the source of the semen found in the rape kit. The fact that rape kits are routinely collected and saved is one reason why a number of convicted rapists have been exonerated due to newly tested DNA technology that did not exist at the time that the accused was tried and convicted.
Today, no one disagrees that DNA is totally reliable in excluding a defendant as the source of the sample. Juries, however, are free to reject DNA evidence despite its scientific reliability.
Restriction Fragment Length Polymorphism (RFLP) Testing
Adenine pairs only with thymine and guanine pairs only with cytosine. The order of the chemical building blocks distinguishes different DNA strands. By measuring the number of repeating patterns of these four chemicals at a particular locus on the strand of DNA, scientists can create a DNA profile. And as the length of the strands differs among different people, a profile of one person's DNA strands will differ from another's. The earliest form of DNA testing looked for repeating patterns and marked the DNA whenever a repeat occurred. The chemicals are identified by their first letters; therefore a possible repeating pattern might look like this: AGCTCAATGC.
By using "restriction enzymes," scientists discovered that they could break DNA into smaller pieces at the places where a tandem repeat ended. RFLP testing involved applying an electrophoresic gel to the DNA sample. The gel would travel until it detected the end of a pattern. By using light, transfer of the pattern, and photography, scientists created a pattern that looks like a bar code in a supermarket. RFLP testing required at least a tablespoon of genetic material for testing. It was also expensive. The earliest court cases used RFLP testing and raised a number of issues about its reliability. Although RFLP testing has largely been superseded by a more efficient method called Short Tandem Repeat (STR) testing, the basic concept of identifying different length alleles is the premise of all DNA testing. Here is how one California court described the process:
When a sample of DNA — usually in the form of hair, blood, saliva, or semen — is left at the crime scene by a perpetrator, a forensic genetic...
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