## Facts

- A storekeeper was attacked and raped while opening her business in Sierra Vista, Arizona.
- Police collected the victim’s clothing and paper towels the victim used to clean herself after the assault.
- A DPS criminalist performed RFLP DNA testing on blood and semen stains from the clothing and paper towels and determined the profile matched Robert Wayne Johnson.
- The State presented expert testimony that the probability of a random DNA match was 1 in 312 million, calculated using the NRC (1992) modified ceiling method and a DPS database of roughly 200 samples from each of four racial groups, described as satisfying Hardy–Weinberg equilibrium.
- Over objection, the trial court admitted the DNA match evidence and the random-match probability statistic; the jury convicted Johnson of sexual assault, and he received an aggravated 14-year prison term.

## Issues

1. Whether expert testimony presenting a numerical random-match probability for an RFLP DNA match was admissible under Arizona’s standards for scientific evidence when calculated using NRC-endorsed methods and a population database claimed to satisfy Hardy–Weinberg and linkage equilibrium assumptions.
2. If admission of the random-match probability statistic was erroneous, whether any error required reversal or was harmless.

## Decision

- The Arizona Supreme Court affirmed the admission of the DNA probability statistic testimony.
- The court held that, on the record presented, the NRC-based modified ceiling method and the DPS database foundation were sufficiently reliable for admission.
- The court affirmed Johnson’s conviction and sentence.
- The court further concluded that any arguable deficiencies in assumptions or presentation did not warrant reversal.

## Legal Principles

- When a DNA profile match is shown, the significance of the match may be explained to the jury through random-match probability statistics if the underlying scientific methodology is reliable and generally accepted.
- NRC-recommended approaches to DNA population statistics, including the modified ceiling method, can provide an acceptable scientific basis for calculating random-match probabilities.
- A proper foundation for DNA probability evidence may include evidence that the loci are treated as independent (linkage equilibrium) and that the population database used to estimate allele frequencies is adequate and consistent with Hardy–Weinberg equilibrium assumptions.
- Challenges to statistical assumptions and database adequacy typically go to weight rather than admissibility when the proponent establishes a reliable methodology and sufficient foundational support.

## Conclusion

The court affirmed a sexual assault conviction after holding that DNA random-match probability testimony (1 in 312 million), calculated using the NRC’s modified ceiling method and supported by testimony about database size and equilibrium assumptions, was admissible and not grounds for reversal.


State v. Johnson, 922 P.2d 294 (Ariz. 1996)