It seems the ongoing court case involving O.J. Simpson has brought a new focus on DNA and court related decisions. Here are answers to the most common questions that come up about DNA and related court decisions.

What is deoxyribonucleic acid (DNA)?

DNA is a molecule that carries a person's "genetic code." It is present in every cell of the body that contains a nucleus, which in humans includes all cells except red blood cells. The DNA in each of an individual's cells is identical regardless of whether the biological source is blood, hair, semen or tissue. Every person's DNA is unique, except for that of identical twins. The typical DNA molecule contains millions of nucleotides, the building blocks that make up the nucleic acid DNA. Each DNA molecule has two chains of nucleotides linked in a formation that twists and looks like a spiral staircase or ladder. The "rungs" of this spiral ladder are the critical components of the DNA molecule for the purposes of DNA profiling. Each rung is commonly referred to as a "base pair" or "base sequence," and the order in which the base pairs appear on the ladder constitutes the genetic code for the cell.

Each person's genetic code, also called the "genome," contains about 100,000 genes and approximately 3.3 billion base pairs. Most people are biologically very similar, and of all these base pairs, only about 3 million (or one percent) differ from one person to another, and the differences are referred to as "polymorphisms." It is these differences, or polymorphisms, that provide the basis for DNA identification and have great significance for DNA forensic analysis.

What are the tests for DNA matching?

There are two tests used in DNA profiling. They are commonly referred to as "RFLP" and "PCR." Restriction Fragment Length Polymorphism (RFLP) compares the sizes (total fragment length) of a polymorphism - the part of the DNA molecule that differs from person to person.

The RFLP test requires a larger DNA sample than the PCR test. For the RFLP test, approximately 5,000 cells are needed. In addition, the sample must be fairly "fresh." That's because the RFLP tests longer fragments than the PCR test, and the longer the fragment the quicker the degrading of the matter. The test can take 12 or more weeks to complete.

In contrast to RFLP, the Polymerase Chain Reaction (PCR) test uses a much smaller sample of around 50 cells, and the cells tested need not be "fresh." The PCR test identifies an area of DNA where individuals tend to vary from each other, then replicates that area thousands of times (which is why it is called a "chain reaction" test.) Once replicated (or, as it is commonly referred to, "amplified"), the DNA is typed through genetic probes. If two samples have the same type, they may have come from the same source.

When first used, the PCR test was less precise than RFLP, because PCR tested only one gene, called the HLA-DQ alpha marker. Hence, the PCR test was often used to exonerate rather than incriminate a suspect. That's because the threshold for excluding a suspect is much lower than that for including or incriminating an individual.

With new developments, however, PCR testing is becoming more precise than it was a few years ago and more popular than RFLP. Today, five more "markers" (genes) have been identified for PCR testing, and the test is now often referred to as the "PCR polymarker" test. Its popularity has increased for several reasons: It can be completed faster than RFLP (in two weeks versus two to three months). A PCR polymarker requires a fraction of the sample that RFLP requires for testing (50 cells versus 5,000 cells).

What is the controversy in the scientific community about DNA matching?

The battle is among population geneticists and forensic scientists. (Population geneticists are concerned with allele frequency in a particular part of the population, such as African-Americans, Caucasians or Hispanics. Alleles are the gene variations responsible for different traits such as hair or eye color.) The essence of the debate surrounds the methods for estimating the population frequencies of specific DNA typing patterns. Opponents of DNA matching claim that the proponents erroneously presume that certain characteristics occur with regularity in a given ethnic group. Thus, the opponents contend that the proponents' probabilities are much higher than reality. The opponents say it will take 10-15 years to assemble data on genetic variation among ethnic groups to support claims of matching by chance.

Have there ever been "false positives" through DNA matching?

In a 1987 study (at the very early stages of DNA analysis before integrity controls existed), there were some false positives. They were due to two errors: incorrect loading of samples into testing machines and contamination of one of the materials used in the test.

There is no current evidence of false positives. The procedures used and integrity controls employed are now considered extraordinarily reliable. The error rates are well under one percent today, and that one percent figure accounts primarily for false exclusions, not false inclusions. That is, a suspect is more likely to be falsely exonerated rather than falsely accused.

Are there any petitions for review on the admissibility of DNA before the United State Supreme Court?

There are no known petitions for review currently before the United States Supreme Court.

What laws will apply to the handling of DNA evidence for the O.J. Simpson Trial?

California law will apply to DNA evidence since the trial is being held in California. California's highest court has yet to accept an appeal challenging the admissibility of DNA evidence. At this time, there are no petitions before the U.S. Supreme Court involving DNA. However, a review of California appellate court decisions give a valuable insight to how courts regard DNA evidence and determine admissibility.

While two appellate courts are split on the issue of admissibility, the trial courts in California almost uniformly admit the evidence and leave the question of what weight to be given it to the jury. In approximately 75 cases where the prosecution sought to use DNA evidence, the trial court did not admit it in only three or four cases. In those cases, the state's case was proven without the DNA evidence.

A summary of the three leading published appellate court decisions followed by a brief discussion of the admissibility standard in California for DNA  evidence will help to understand the logic of rulings that may occur during the upcoming Simpson trial.
 

The first case to consider the admissibility of DNA evidence was People v. Axell in 1991. Axell was heard by the Second District which covers the greater Los Angeles area. The split in authority is between the First District (San Francisco) and the Second District (Los Angeles) with the Second District finding the evidence admissible in 1991 and the First District, in two successive years, holding the opposite.

Case #1 
People v. Axell, Second District, decided October 29, 1991. Lynda Patricia Axell was convicted of robbing and murdering a man in a Ventura restaurant adjacent to Axe II' s place of employment. Long hairs with follicles attached were recovered from the scene. The prosecution used Cellmark Laboratories for a DNA comparison of the hair samples to blood samples extracted from Axe II after her arrest. A DNA match, with the probability of 1 in 6 billion (in the Hispanic population), was introduced into evidence at trial following a lengthy admissibility hearing.

Axell was the first appellate case in California to grapple with the admissibility of DNA typing evidence. The Court held that DNA evidence met the legal requirements for admissibility of novel scientific evidence and met the foundational requirements for the calculation of statistical probability employed by Cell mark, the testing laboratory.

Case #2 
People v. Barney and Howard, First District, decided August 5, 1992. This appeal consolidated two unrelated cases which both challenged the admissibility of DNA evidence.

Ralph Edward Barney kidnapped, raped, and robbed at knife point his female victim. A DNA comparison by Cell mark of semen recovered immediately following the crime matched Barney with a probability of 1 in 7.8 million in the black population.
 

Kevin O'Neal Howard strangled and beat to death his landlord, Octavia Matthews, who had begun eviction proceedings against him for non-payment of rent. At the time of his arrest, Howard had a fresh cut on one of his fingers. A DNA comparison by the FBI of blood samples recovered from the crime scene matched Howard's with a probability of 1 in 200 million in the African-American population. 

The trial court in each case admitted the DNA evidence and each defendant challenged it on appeal. The Court held that the standard was met for the processing and matching steps of DNA analysis but was not met for the statistical analysis step because of an absence of a general scientific acceptance as to the process used to calculate the statistical significance of a match of DNA patterns. The Court further held that, although the DNA evidence was improperly admitted, it was harmless error in light of the overwhelming evidence of defendants' guilt. The court noted "a match of DNA patterns is a matter of substantial significance."

Case #3 
People v. Wallace, First District, decided March 25, 1993. On three separate occasions, Jeffrey Allen Wallace raped and assaulted four women, and assaulted a fifth, using a distinctive method when committing the crimes. A DNA comparison of semen recovered from three of the victims with a blood sample from Wallace revealed a match with the probabilities of one in 26 million in the Caucasian population. The prosecution expert, Dr. Sensabaugh, declared the probabilities as "in excess of one in a million" stating that "once we go beyond one in a million, we don't really have a good sense of what the numbers mean".

The court noted that since only eight months had passed since the Barney decision, it was still too soon to confirm that the statistical calculation proposed by the National Research Council's report would receive general acceptance resulting in future admissibility. The Court found it an error to have admitted the DNA evidence, but as it did with Barney, the Court found the error harmless.

Perhaps shedding some light on its reluctance to find DNA evidence inadmissible, the Court went on to state:

This is no time for purist insistence that DNA evidence should be admitted on one's own terms or not at all. As Dr. Sensabaugh clearly stated, it matters little to a jury whether the odds are one in one million or one in 26 million. Either estimate is devastating to the defendant. Our hope is that the key players in this dispute will take their cue from Dr. Sensabaugh and agree to a compromise on statistical calculation. Otherwise, they risk preventing any general acceptance at all, thus precluding the admissibility of DNA analysis evidence.

The "Frye" Standard

California currently applies the "Frye" standard for admissibility of new and novel scientific evidence. Frye is a 1923 United States Supreme Court lie detector case which set forth the rule that new and novel scientific evidence is not admissible unless it is "generally accepted in the scientific community."

Those courts in California that have not admitted DNA evidence (including the First District) rely on the fact that the scientific community (forensic scientists and population geneticists) have not agreed on the statistical probabilities of a chance match. Those courts that have admitted such evidence do so on the grounds that the statistical dispute is a matter of weight for the jury to consider; not a question of admissibility and that not to admit it would keep relevant and probative evidence from the jury. (The experts on both sides agree that the scientific theory underlying DNA profiling and the techniques employed are sound. They only disagree on what method should be used to calculate the probabilities of a chance match.) 

United States Supreme Court Ruling 

Last year, the United States Supreme Court overruled Frye in Daubertv. Merrell Dow Pharmaceuticals, Inc. The Court set a new standard for the admissibility of new and novel scientific evidence that comports with Federal Rule 702 and is referred to as the "reliability" standard versus the "general acceptance" standard. Daubert states that "general acceptance" is only one of several factors one must demonstrate to lay a proper evidentiary foundation. Daubert, like Rule 702, looks at other factors such as the reliability of the scientific test itself, the novelty of the technique, the rate of laboratory error, the existence and application of standards governing use of the method, the presence of scientific literature concerning the field of analyses, the potential of juror confusion, and the probative value of the evidence.

In California, oral arguments were recently heard (August 30, 1994) before the California Supreme Court in the case of People v. Leahy, a drunk driving case. One of the issues to be argued is whether California should continue to be a Frye state or whether it should become a Daubert state. If the case is argued as scheduled and if the Court renders a decision in its usual time period, it will be December before there is a answer to the question. Ironically, the decision may come in the midst of the Simpson Trial.