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Expert Testimony of STRmix Method of DNA Analysis OK

The Gist of this Article: The STRmix method of DNA analysis is reliable under the Kelly test of general scientific evidence admissibility.  The following case summary explains the challenge to its methodology and why it was found reliable.        
Alvin Larry Davis, age 72, was charged in San Joaquin County Superior Court with the violent sexual assault and killing of an 84-year-old woman, Hazel Dingman, inside her large home in Stockton and during a burglary that took place on July 5, 2012. 

The house sat on five acres.  Ms. Dingman lived alone.  She had been tied up and brutally attacked.  Her home was in disarray and items were missing. 

The first trial ended in a mistrial because the jury could not reach a unanimous verdict.  The critical issue was identity.  The defense theory was that Davis was not the person who murdered Dingman.  There were many homeless people who lived near her house.  Ms. Dingman also had a brother, 91 years old, who had a stepson, Edward Wingate, who often came to the property with his friends to fix cars and fish.  They frequently used methamphetamine while there.

The People relied on a variety of evidence connecting Davis to the murder, including DNA evidence from a bloody shoelace found next to Dingman’s body.  Davis had a Cadillac Escalade that Ms. Dingman’s brother’s stepson was repairing, so Davis had been by the Dingman home 10 to 12 times while his Escalade was being worked on, but he never went in the home.

In a second trial, however, the jury found Davis guilty of first degree murder (Penal Code § 187(a)) and forcible sexual penetration with a foreign object, a cane (Penal Code § 289(a)(1)(A)).  The jury also found true the enhancement allegations, including that he tied or bound the victim (Penal Code § 667.61(e)(5)), personally inflicted great bodily injury on the victim (Penal Code § 667.61(d)(6)), sexually penetrated the victim during the commission of the burglary (Penal Code § 667.61(d)(4)), and knew or reasonably should have known that the victim was 65 years of age or older (Penal Code § 667.9(a)).

In a bifurcated proceeding, the trial court found that defendant had two prior convictions that qualified as strikes (Penal Code §§ 667, 1170.12).  The judge, George J. Abdallah, Jr., sentenced Davis to life in prison without the possibility of parole.

On appeal to the Third Appellate District Court, Davis contended the judgment had to be reversed because of several issues, but this article’s scope will narrow down to Davis’ argument that Judge Abdallah improperly admitted expert testimony based upon the trademarked STRmix methodology, which is a method of deoxyribonucleic acid (DNA) analysis.

Davis argued that the STRmix evidence should have been excluded under the test for the admission of new scientific evidence established by the California Supreme Court in People v. Kelly (1976) 17 Cal. 3d 24, abrogated by statute on another point as explained in People v. Wilkinson (2004) 33 Cal.4th 821, 845-848.

The Third Appellate District Court concluded that the trial court did not err in determining that the STRmix method of DNA analysis is generally accepted as reliable by the relevant scientific community such that expect testimony relying on the method satisfied the first requirement of the Kelly test.

The Third Appellate District began its analysis with a brief overview of what DNA is and how useful it can be to link a piece of evidence from a crime scene or a victim to a suspect.  The court explained how polymerase chain reacting (PCR) testing is used to amplify targeted loci of the sample of DNA by replicating the process by which DNA duplicates itself naturally.  It also explained how Short Tandem Repeats, or STR’s, are a group of loci which are used to type and compare the DNA.

In this case, Pin Kyo, a senior criminalist from the California Department of Justice (DOJ) performed DNA testing on various items in defendant’s case, including the blood shoelace and created DNA profiles utilizing the PCR-STR method of DNA analysis.

Davis did not challenge the PCR-STR method of DNA analysis that Kyo used to create the DNA profiles which has gained general acceptance in the scientific community for the analysis of forensic evidence samples, including DNA from multiple contributors.

However, Kyo determined that there were three contributors to the DNA on the bloody shoelace found next to Dingman’s body, at least one of which was male.  However, because the DNA mixture was too complex for her to interpret, Kyo sent the shoelace to another DOJ laboratory for application of the STRmix methodology, which is a method of DNA analysis that involves the use of probabilistic genotyping computer software to aid in the interpretation and evaluation of forensic evidence that contain a mixture of DNA from multiple contributors.

The People then satisfied all three prongs of the Kelly test for expert testimony to rely upon the STRmix technique.  First, it showed that the reliability of the new technique has gained general acceptance in the relevant scientific community.  Second, the expert testifying to that effect was qualified to give an opinion on the subject.  Third, the correct scientific procedures were used. 

Therefore, as to this portion of Davis appeal, his arguments for reversal were denied.

The citation for the Third Appellate District Court ruling discussed above is People v. Alvin Larry Davis (3rd App. Dist., 2022) 75 Cal. App. 5th 694, 290 Cal. Rptr. 3d 661.

For more information about expert testimony and “junk science,” please click on the following articles:
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