Friday, July 8, 2011

Confusing DNA Samples with DNA Profiles?

It is tough for lawyers to get science right. I say this not to denigrate lawyers—I am one myself—but to stress the importance of taking the time and effort to communicate the scientific facts clearly so that the value judgments are persuasive. An article attacking the constitutionality of an Arkansas law on DNA sampling from arrestees illustrates this point. In “Step Out of the Car: License, Registration, and DNA Please,” Associate Professor Brian Gallini of the University of Arkansas School of Law, gives an account of DNA profiling that makes it appear that the process of forensic DNA profiling reveals “the totality of a person’s genetic makeup” to arrive at an identification profile. At least, that is how the following exposition of DNA profiling for identification could be read:
[E]ven the layperson knows that taking a DNA sample requires an intrusion into the body, which thereafter reveals the totality of a person’s genetic makeup. ... Although courts have characterized DNA swabs as only “minimally intrusive,” they do so without recognizing ... the intrusion upon the arrestee’s interest in keeping the information revealed by a DNA sample private. From a buccal swab, the state obtains an analyzable sample of an arrestee’s DNA. That, in turn, allows the state to perform a polymerase chain reaction procedure (PCR), which involves replicating the DNA sample. This replication then allows the tester to look at “short tandem repeats” (STR). At this stage, the STRs reveal specific areas of DNA known as “loci.” In total, the tester is looking to isolate thirteen different loci in order to identify an individual’s exact genetic makeup. Once complete, the sample potentially “provides the instructions for all human characteristics, from eye color to height to blood type.”
What is wrong with this picture? Let me count the ways:
  1. PCR does not replicate the DNA sample. Human cells can replicate the full nuclear genome, but PCR can only replicate short stretches of DNA from targeted locations—the loci.
  2. Replication itself does not allow the tester to look at STRs. Visualization or ascertainment comes later.
  3. STRs do not “reveal specific areas of DNA known as ‘loci.’” An STR is a certain type of DNA sequence that occurs at, well, an STR locus. PCR primers used in forensic identification amplify only the sequences at these loci. The rest of the genomes remains terra incognito.
  4. The tester is not seeking “to identify an individual’s exact genetic makeup.” Rather, the laboratory is seeking to ascertain a small number of variations that are not in genes (or not in the exons of genes).
  5. The physical sample was complete before it was typed. “Once complete,” the tiny profile cannot possibly “provide[] the instructions for all human characteristics, from eye color to height to blood type.” The STR typing never gives any instructions for phenotypes.

Do these corrections mean that samples could not be used to gain information about human phenotypes such as eye color? Of course not. Eye color is a phenotype that can be deduced (in some instances) from genotyping. But such genotyping is not STR profiling.

And how much would it invade your privacy if a laboratory technician were to figure out your eye color in this roundabout way—instead of looking you in the eye? But that’s another story, and I have argued elsewhere against indefinite sample retention.


Brian Gallini, Step Out of the Car: License, Registration, and DNA Please, 62 Ark. L. Rev. 475 (2009)

Crossposted from the Double Helix Law blog.

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