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January 28, 1994



Rehearing Denied March 9, 1994. Released for Publication March 18, 1994. Petition for Leave to Appeal Denied October 6, 1994.

McNAMARA, Egan, Rakowski

The opinion of the court was delivered by: Mcnamara

JUSTICE McNAMARA delivered the opinion of the court:

Pursuant to Illinois Supreme Court Rule 604(a)(1) (134 Ill. 2d R. 604(a)(1)), the State appeals from an order of the trial court ruling inadmissible scientific evidence offered against defendant, Vernon Watson, who was charged with aggravated criminal sexual assault, armed robbery, and aggravated kidnapping. Prior to trial, defendant filed a motion in limine seeking to exclude the results of a DNA profiling analysis performed by the Federal Bureau of Investigation (FBI) which indicated that defendant's DNA matched that of the assailant. The trial court conducted an extensive Frye hearing ( Frye v. United States (D.C. Cir. 1923), 54 App. D.C. 46, 293 F. 1013) to determine whether the evidence would be admissible at trial. The Frye court held that expert opinion based on a scientific technique is inadmissible unless the technique is "generally accepted" as reliable in the relevant scientific community. The trial court here ultimately ruled that, while the methodology used in declaring a DNA match was generally accepted within the relevant scientific community, satisfying the requirements set forth in Frye, the procedures employed in calculating the statistical probability of a random match were not generally accepted in the relevant scientific community. On this basis, the trial Judge granted defendant's motion to exclude the results of the DNA profiling analysis at trial. The State has appealed.

Defendant was charged with the aggravated criminal sexual assault, armed robbery, and aggravated kidnapping of C.A., a 24-year-old woman who was attacked on May 25, 1989 near 1900 West 91st Street in Chicago as she was walking to a nearby commuter train station. In the course of their investigation, Chicago police officers recovered evidentiary items containing bodily fluid, including semen, from the body and clothing of the victim.

The police submitted the evidentiary samples to the DNA Analysis Unit of the FBI, along with blood samples of defendant and the victim. At the specific request of the FBI, the police also submitted a blood sample of the victim's husband. The FBI analyzed the samples using the restriction fragment length polymorphism (RFLP) technique. The analysis indicated that the DNA profiles in the evidentiary samples "matched" the DNA profile of defendant. The FBI then calculated an estimate of the probability that a person chosen at random from the Black population would match this DNA profile. That probability was determined to be one in ninety million.

At the Frye hearing, which spanned several weeks and generated over 1,500 pages of transcript, the court heard expert testimony from three witnesses for the State and four for the defendant. The State's witnesses and their backgrounds are as follows: (1) Dr. Charles Strom, Director of the DNA Laboratory at Illinois Masonic Hospital in Chicago, qualified as an expert in the areas of molecular genetics and molecular biology; (2) Dr. Harold Deadman, supervising special agent in charge of the FBI's DNA Analysis Unit, qualified as an expert in forensic chemistry and called to offer his expert opinion regarding DNA profiling in the field of forensics, and, as a rebuttal witness; (3) Dr. Michael Conneally, Distinguished Professor of Medical Genetics and Neurology at Indiana University Medical Center, qualified as an expert in the field of human population genetics. The following witnesses testified for defendant: (1) Dr. Randell Libby, Assistant Professor of Genetics at the University of Washington, qualified as an expert in the fields of molecular biology, molecular genetics and forensic DNA analysis; (2) Dr. Jerry Coyne, Associate Professor of Ecology and Evolution at the University of Chicago, qualified as an expert in the areas of population genetics, evolutionary biology and biostatistics; (3) Dr. Seymour Geisser, Director of the School of Statistics at the University of Minnesota, qualified as an expert in biostatistics and probability theory; and (4) Dr. Lawrence Mueller, Associate Professor of Ecology at the University of California at Irvine, qualified as an expert in the fields of population genetics and evolutionary biology.

The court also considered some 68 exhibits consisting primarily of published articles and reports pertaining to DNA testing as well as opinions issued by courts outside this jurisdiction which have addressed the admissibility of DNA profiling evidence in a criminal trial.

Following the hearing, the trial court issued an order granting defendant's motion to exclude the DNA evidence along with a scholarly and detailed 36-page opinion explaining the reasons for its decision. In particular, the trial court found that, while the scientific theory underlying DNA testing and the RFLP technique used by the FBI to determine a match are generally accepted in the relevant scientific community, the methodology used to estimate the probability of a random match in the relevant population was not generally accepted. The trial court concluded that without the probability assessment, the jury would not know what to make of the fact that the DNA patterns "matched." Accordingly, the State's evidence of a match as well as the statistical assessment of the match were held inadmissible at trial.

In addressing the State's contention that the trial court erred in ruling the DNA evidence inadmissible at trial, a basic understanding of the theories and procedures involved in DNA profiling is essential to comprehend the legal issues surrounding its use as evidence in court. Therefore, we shall first consider the general nature of the particular evidence the State sought to have admitted. Our Discussion of DNA and DNA profiling is derived primarily from testimony given at the Frye hearing and from a comprehensive and long-awaited report dealing with forensic DNA methodologies entitled "DNA Technology in Forensic Science" (hereafter referred to as "NRC Report"), which the National Research Council published in 1992 and to which both parties refer in their written briefs submitted in this court.

DNA profiling involves two distinct procedures. First, RFLP analysis determines if there is a match. A "match" does not mean that the suspect is with certainty the source of the genetic material found at the crime scene or on the victim, but only that the suspect cannot be eliminated as a potential source. Even if there is a perfect matchat four or five different loci, there is still a possibility that the two samples came from different people whose DNA patterns at those particular loci are indistinguishable. Thus, the second procedure, calculation of the probability of a random match, generates a ratio which accompanies a match, the purpose of which is to express the statistical likelihood that an unrelated person chosen at random from a particular population could have the same DNA profile as the suspect.

For criminal cases, DNA testing is of very recent advent. In October 1988, a court of review first considered the admissibility of DNA testing in the criminal context. (See Thompson and Ford, DNA Typing: Acceptance and Weight of the New Genetic Identification Tests, 75 Va. L. Rev. 45, 46 n. 4 (1989) (citing Andrews v. State (Fla. App. 1988), 533 So. 2d 841)) (hereafter Thompson and Ford). In the years following Andrews, courts in more than 40 states have considered DNA evidence in hundreds of cases. NRC Report at 21-22.

Deoxyribonucleic acid, DNA, is found in the chromosomes contained in the nuclei of cells. It provides the genetic blueprint which determines the physical structures and individual characteristics of every living organism. In humans, DNA exists in all cells that have a nucleus, including white blood cells, cells surrounding hair roots, and cells found in sperm and saliva.

With exceptions not relevant here, DNA does not vary within an individual, i.e., the DNA contained in one cell in an individual will be identical to the DNA contained in every other cell of that individual. The important feature of DNA for forensic purposes is that, with the exception of identical twins, no two individuals have the same DNA structure.

A molecule of DNA is shaped like a double helix and resembles a twisted ladder or spiral staircase. The sides of this ladder, which are composed of phosphate and sugar molecules, are connected by "rungs" made up of pairs of molecules called "bases" (also referred to as nucleotides). The critical components of the ladder are these rungs. There are four types of bases in the DNA molecule, and they are designated as adenine (A), guanine (G), cytosine (C), and thymine (T). The bases bond in predictable patterns, A to T and C to G. This strict complementary pairing means that the order of the bases on one side of a DNA ladder will determine the order on the other side. The order in which these base pairs appear on the DNA ladder constitutes the genetic code for the cell. This code carries the necessary information to produce the myriad proteins which comprise the human body. Because human beings share more biological similarities than differences, well over 90% of the DNA molecules, or base pair sequences, in each human are the same. Certain sections of the DNA ladder, however, take different forms in different individuals. These areas of variation, called "polymorphisms," provide the basis for DNA identification and produce great significance for forensic testing.

A sequence of base pairs responsible for producing a particular protein or characteristic is called a "gene" (e.g., each person has a gene for the production of eyes). Some genes are polymorphic and may have two or more different versions called "alleles" (e.g., blue-eyed allele, green-eyed allele). The total fragment length of a polymorphism is called a Restriction Fragment Length Polymorphism (RFLP)--hence the name given to the FBI's matching procedure--and its length is determined by the number of repeat core sequences of base pairs, which are called Variable Number Tandem Repeats (VNTRs). A particular region on the DNA molecule where a specific VNTR occurs is called a "locus." A locus is considered polymorphic when the number of VNTRs varies from individual to individual.

Because it is impractical to examine all the polymorphic regions of the DNA molecule, DNA profiling focuses on several highly polymorphic or "hypervariable" segments of DNA. Different people will have the same VNTRs in a particular hypervariable locus, but the loci will differ in length because varying numbers of the VNTRs are linked together. One person, for instance, may have a particular locus in which a given core sequence repeats only 10 times, whereas that same locus in another person may contain the same VNTR that repeats 100 times. Although a person may not have a unique polymorphic area at any one locus, the frequency with which two people will exhibit eight or 10 identical alleles at four or five different loci is extremely low. (There are two alleles which occupy the same locus on a DNA molecule; one is inherited from the mother and one from the father. When the alleles that comprise a pair differ, the individual is said to be "heterozygous" for that allele. When the maternal and paternal alleles in a pair are the same, the individual is "homozygous" for that allele.)

DNA analysis is generally performed by disassembling the ladder in one of several ways. The FBI, as well as two commercial laboratories, Cellmark and Lifecodes, uses the RFLP method of analysis. The operative steps of RFLP analysis, as utilized by the FBI, are outlined below:

1. Extraction of DNA. The DNA is first extracted from a sample of certain tissue or bodily fluid, such as blood or semen, by using chemical enzymes and is then purified.

2. Restriction or Digestion. The DNA is then "cut" into smaller fragments with chemical scissors called restriction enzymes. Theseenzymes recognize certain base pairs and sever the DNA molecule at specifically-targeted base pair sites to produce RFLPs.

3. Gel Electrophoresis. The cut fragments of DNA molecules are next placed in an agarose gel which is later electrically charged to sort the fragments by length. The electric current causes the fragments to migrate through the gel. The distance traveled depends upon the length of the fragment; the shorter fragments, because they are lighter, will travel further in the gel. Fragments of known base pair lengths, called molecular weight markers, are placed in separate lanes to allow the measurement of RFLPs in units of base pairs. Several different samples are run on the same gel, but in different lanes.

4. Southern Transfer or Blotting. Because the agarose gel is difficult to work with, the RFLPs are transferred to a more functional surface by a method called "Southern transfer." A nylon membrane is placed over the gel, causing the RFLPs to move onto the membrane. The RFLPs then become permanently affixed to the membrane, referred to as a "blot," in the same pattern as in the gel. Also during this step, a denaturization process severs each double-stranded DNA fragment into two single strands--one inherited from the mother and one from the father.

5. Hybridization. In this step, a genetic probe is used to locate a specific locus of a polymorphic region of the DNA on the blot. A genetic probe is a single-stranded segment of DNA designed to complement a single-stranded base sequence that is associated with a particular locus on a chromosomal pair. The probe will bond with any single-stranded fragments containing that particular base sequence. The normal result is that the probe will bind to DNA fragments, or RFLPs, at one or two locations in each lane, depending on whether the individual is homozygous or heterozygous for that particular allele. The genetic probe is tagged with a radioactive marker, which attaches to the probe and emits radiation without altering the function of the probe. The marker is used to determine the probe's position on the blot after it hybridizes with a polymorphic segment.

6. Autoradiography. Next, the nylon membrane is placed in contact with a piece of x-ray film where the radioactive probes expose the film at their respective locations. Black bands appear where the radioactive probes have bonded to the RFLPs, producing a DNA "print." Typically, each probe will expose one or two bands for each DNA sample, which reflects the maternal or paternal contributions to the individual's DNA profile. The position of each band indicates the location of a polymorphic segment on the blot. Location, in turn, indicates the length of the DNA fragment that contains the ...

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