State v. Johnson

An Arizona jury convicted Robert Johnson (defendant) of sexual assault for the 1991 rape of a woman who was attacked in her shop. The trial court admitted expert testimony from Terry Hogan, a criminalist at the Arizona Department of Public Safety (DPS) crime laboratory. Hogan testified that he extracted DNA from semen stains found on the woman’s clothing and matched that DNA to Johnson’s DNA. Hogan calculated that the probability of a similar DNA match occurring randomly was one in 312 million. Hogan’s testimony heavily relied upon DNA-comparison standards outlined in a 1992 National Research Council (NRC) report by highly respected scientists. According to the NRC report, scientists agreed that DNA-match probability could be considered truly random if the comparison method, calculation method, and comparison database were in linkage equilibrium and Hardy-Weinberg equilibrium. Linkage equilibrium is the assumption that within any DNA sample, each allele (i.e., a portion of a DNA strand that determines a genetic trait) occurs independently from other alleles. Hardy-Weinberg equilibrium is the assumption that within any population, people choose their mates randomly, which produces a random frequency of any allele. The NRC report noted that the generally accepted DNA-sample comparison method, restricted-fragment-length polymorphism (RFLP), ensured linkage equilibrium by comparing only alleles that scientists agreed occurred randomly and had no statistical relationship to each other. The NRC report also endorsed a modified-ceiling calculation method that made conservative, race-neutral statistical adjustments that generally favored defendants. The NRC report stated that a DNA-sample database sufficiently represented the general population if it drew DNA samples from at least 150 unrelated people across at least three different racial populations. Hogan testified that his evidence was based on (1) RFLP, (2) the modified-ceiling method, and (3) the DPS database, which was in Hardy-Weinberg equilibrium and included about 200 samples from each of four racial groups. Johnson appealed from his conviction, and the Arizona Court of Appeals affirmed. The Arizona Supreme Court granted Johnson’s petition for review to consider whether the trial court had properly admitted Hogan’s testimony.