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DNA Decision

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On July 7, the California Supreme Court decided "a narrow, but important, question" on presenting DNA match probabilities to the jury. When the defendant's DNA profile matches the crime scene sample, the expert typically calculates the probability that such a match could occur at random. Because the markers used vary in their frequency in different racial groups, the probability can vary by race. This is not a problem when the perpetrator's race is established by independent evidence, such as the testimony of the victim in a rape case. However, there is a question of how to handle these numbers in crimes where there is no surviving witness.

People v. Wilson, S130157, is a noncapital murder case. The opinion is by Justice Ming Chin, who has been at the forefront of DNA issues for some time. The opinion confirms that when the race of the perpetrator is unknown, it is error to simply calculate the probability of a random match in the defendant's own racial group. However, it is proper to calculate the probability separately for all the primary racial groups in the area and present them to the jury. In this case, "Defendant's genetic profile would be expected to occur in one of 96 billion Caucasians, one of 180 billion Hispanics, and one of 340 billion African-Americans."

When the numbers are this large, it seems pointless to squabble about the differences among racial groups. If a number is astronomical, it doesn't matter if it is the moon, Pluto, or Alpha Centauri. On the least of these numbers, the prosecution has disproven the "null hypothesis" of a random chance match far beyond a reasonable doubt, and the only possible defense is to come up with some other explanation for a match that is consistent with innocence. Giving the jury the numbers for all the groups available seems as good a solution as any.

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I wonder how the DNA of the African American population differs from the DNA of West African (or other populations of Africans), given the admixture of other populations in African-Americans (Caucasian and American Indian).

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