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November 15, 1996


Appeal from Circuit Court of Adams County. No. 94CF182. Honorable Dennis K. Cashman, Judge Presiding.

Released for Publication December 17, 1996. As Corrected January 3, 1997.

Honorable Robert J. Steigmann, J., Honorable Robert W. Cook, P.j. - Concur, Honorable Frederick S. Green, J. - Concur. Justice Steigmann delivered the opinion of the court.

The opinion of the court was delivered by: Steigmann

The Honorable Justice STEIGMANN delivered the opinion of the court:

A jury found defendant, Kerry L. Pope, guilty of one count of aggravated criminal sexual assault (720 ILCS 5/12-14 (a)(2) (West 1992)) and one count of aggravated battery of a child (720 ILCS 5/12-4.3(a) (West 1992)), for which he was sentenced to consecutive terms of 60 years' and 30 years' imprisonment, respectively.

Defendant appeals, arguing that (1) the trial court erred by admitting deoxyribonucleic acid (DNA) evidence because the polymerase chain reaction (PCR) amplification-based procedures for DNA identification are not generally accepted in the scientific community; and (2) the prosecutor's improper closing argument deprived him of a fair trial. We affirm.


Prior to trial in August 1994, defendant sought a continuance to conduct a Frye hearing (see Frye v. United States, 54 App. D.C. 46, 293 F. 1013 (D.C. Cir. 1923)) to determine the admissibility of DNA test results based on PCR analysis. Because the Illinois appellate courts had not ruled on the admissibility of such evidence, the trial court granted the motion. Before reviewing the evidence presented at the Frye hearing, we provide background information on both DNA structure and PCR analysis.

A. DNA Structure and Identification

The following background information is taken from Thomas M. Fleming, Annotation, Admissibility of DNA Identification Evidence, 84 A.L.R.4th 313 (1991), unless otherwise noted. The DNA molecule is a long, threadlike structure resembling a twisted ladder packed into the chromosomes in every nucleated cell. Each side of the ladder is composed of a chain of sugars and phosphates, while the rungs attached to them consist of pairs of molecules called bases. While most sections of this chain of bases are largely the same among humans, certain sections are variable. Thus, a gene--the sequence of bases responsible for producing a particular protein--may be polymorphic, having two or more possible variations called alleles. Because of polymorphisms in human genetic structure, no two individuals (except for identical twins) have identical base sequences throughout their DNA.

The PCR amplification method of DNA identification, first devised in 1985, is one of two kinds of DNA tests most commonly used for identification. The other method is restriction fragment length polymorphism (RFLP) analysis, which our supreme court recently held to be generally accepted in the relevant scientific community. People v. Miller, 173 Ill. 2d 167, 188, 670 N.E.2d 721, 731, 219 Ill. Dec. 43 (1996). By 1990, the PCR method was one of the most widely used techniques in medical and biological research. People v. Lee, 212 Mich. App. 228, 264, 537 N.W.2d 233, 250 (1995). PCR-based techniques allow testing of DNA samples that have degraded as well as samples that are very small, such as a single strand of hair.

In the PCR-based DQ-Alpha procedure, an analyst looks at one particular gene, the DQ-Alpha gene, which is genetically inherited and appears in varying forms in different people. Lee, 212 Mich. App. at 264-65, 537 N.W.2d at 250. Six readily detectable alleles are present in the DQ-Alpha locus, and they are identified with the following numbers: 1.1, 1.2, 1.3, 2, 3, and 4. These alleles are combined in pairs in each individual, and the six combinations result in 21 possible "genotypes," each of which appears in varying proportions within the population.

In the PCR DQ-Alpha procedure, DNA is extracted from a sample, purified, and added to a buffer solution containing chemical primers (small pieces of DNA that recognize the four bases) and an enzyme called "TAQ polymerase." The solution is then modified (denatured) by being placed in a thermal cycler which cycles it through several successive temperature plateaus. During each cycle, the primer targets a specific gene and will bind to the genetically complementary portion of the DNA; further, TAQ polymerase works to copy the targeted gene. After 30 to 40 cycles, the DNA strand containing the targeted gene has been amplified billions of times.

The amplified DNA is then flooded over a nylon membrane onto which have been dotted a number of "allele-specific" probes, each of which is designed to recognize one sequence of the targeted gene. This may result in a color reaction and a visible dot on the membrane wherever a probe has identified one of the alleles. A blue dot is a match; a blank is a nonmatch. The amplified DNA may then be typed for the various DQ-Alpha genotypes. This process is referred to as the reverse dot blot hybridization or the blue-dot procedure.

If the DQ-Alpha genotype of a suspect is different from that of the evidence sample, the suspect is excluded as a source of the evidence. If the suspect and the evidence sample have the same genotype, then the suspect is included as a possible source. The probability that some other, unrelated individual would also match the evidence sample is equal to the frequency of the genotype in the relevant population.

B. The Frye Hearing

At the Frye hearing, the trial court determined that Dr. Jenifer Ann Lindsey, a Federal Bureau of Investigation (FBI) special agent and a forensic serologist assigned to the DNA analysis unit, could testify as an expert in forensic serology and DNA analysis. Dr. Lindsey explained that the FBI uses two techniques based on PCR analysis--namely, DQ-Alpha typing and polymarker typing. The FBI has been using the DQ-Alpha typing technique since 1992 and the polymarker typing technique since August 1994 (after examining the polymarker technique "for well over a year" through validation and case studies).

Dr. Lindsey testified in detail regarding the PCR DQ-Alpha typing procedure. She specifically noted that FBI analysts perform 32 cycles in the thermal cycler to replicate the targeted gene. Dr. Lindsey also testified regarding the PCR-based polymarker typing procedure. She stated that in polymarker typing, an analyst simultaneously amplifies the DQ-Alpha gene as well as five additional gene regions. Dr. Lindsey also explained the FBI's quality control measures (designed to prevent contamination) and control amplification procedures.

Dr. Lindsey stated that she completed 50 case studies comparing polymarker typing results and RFLP results and found that in all 50 cases the ability to discriminate between two different people was the same for the polymarker and the RFLP procedures. The case studies revealed that some people who were excluded as sources by RFLP testing were included as possible sources by DQ-Alpha typing. DQ-Alpha typing and polymarker typing together provide a power of discrimination (the ability to differentiate between individuals based on the DNA test results) of over 99%. Dr. Lindsey stated that DQ-Alpha typing is being used by many private laboratories, and polymarker typing was first put on the market in January 1994. She also opined that both DQ-Alpha typing and polymarker typing are generally accepted in the scientific community.

Dr. Lindsey further testified regarding the FBI's method of calculating the statistical probability of a random match. By using the product rule, the FBI estimates the statistical probability of a random match between the DNA sample taken from the crime scene and the defendant's DNA sample. In making this estimate, the FBI compares the DNA samples to a previously constructed population database. Because the PCR polymarker systems are two allele or three allele, the FBI may rely on a smaller database. Thus, the PCR Caucasian database contains only 145 individuals. For the polymarker typing procedure, the FBI multiplies the various alleles' frequency rates times each other to determine the statistical probability of a random match.

Regarding DQ-Alpha typing, Dr. Lindsey stated that the FBI tested 500 individuals from four different population groups ("Caucasian," "Black," "Southwestern Hispanic," "Southeastern Hispanic") and determined their DQ-Alpha genotypes. The FBI then added a database developed by Cetus Corporation (containing 237 individuals) because it was statistically similar to the FBI database. Dr. Lindsey stated that to determine the frequency of a particular DQ-Alpha genotype, the FBI uses the actual count method. For example, if an individual had a DQ-Alpha genotype of 2,2, an analyst would look to the database to determine how often that particular genotype occurred within the "Caucasian" ...

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