The DNA molecule, present in every cell in our bodies, is a chain of molecules, sugars, and phosphates. The order in which these components arrange themselves in the chain is, statistically speaking, unique to every individual (except for identical twins). The DNA molecule is the same in every cell, and remains constant (with slight, rare variations) during an individual’s life. Scientists can harvest a person’s DNA molecule from any cell, be it tissue, blood, sweat, semen, bone, saliva, and so on. Once the DNA ladder is analyzed, it can be compared to another molecule’s DNA. Forensic scientists perform such comparisons in order to:

• Link a suspect to a crime or exonerate someone, where biological evidence has been left at the crime, by comparing the suspect’s DNA to that left at the scene
• Determine parentage, for purposes of paternity, immigration, and other cases, and
• Identify human remains, when visual or dental determinations are not possible or conclusive.

DNA identification is extraordinarily reliable, though it can be challenged, as explained below.

How is DNA Tested and Compared?

In the lab, scientists extract an amount of DNA from each biological sample, which must be sufficiently large—a quarter-size blood stain, or a dime-sized semen stain. The DNA molecule itself must be intact, not in pieces that are too small to analyze. Then, each sample’s band pattern (its sequence) is compared to the other. If the bands match, scientists consult a national database of band patterns, and determine the statistical chances that the match is a coincidence. In court, you’ll hear the scientist testify that the odds of the match being coincidental are, for instance, many millions-to-one, leaving the jury with the evidence it needs to accept the match as correct. (Another type of analysis can be performed on a smaller sample, such as a single hair, with results that may be less convincing.)

Factors Affecting DNA Test Results

The process of collecting, testing, and interpreting DNA samples is not foolproof. Samples are collected literally “in the field,” not in pristine laboratories. Many factors can influence the reliability of the results. They include:

• Degradation due to age, environment, contamination. The sample collected at a crime scene or elsewhere may be old, dried-up, soggy, or sullied by grease, dirt, or other contaminants. These are factors that may break the DNA into small fragments, reducing the clarity of the bands or their sizes.
• Variations in interpretation of the bands due to human subjectivity. In the end, a human compares the samples’ bands and declares a match—or not. When a sample has been degraded, the analyst lacks a crisp print, which makes it possible to mistakenly declare a match.
• Lack of consensus as to the basis for statistical comparisons. A match derives its persuasive force from the analyst’s ability to tell the judge or jury that the odds of a coincidental match are astronomical. Yet within the scientific community, a debate exists as to the basis for these statistical pronouncements.
• Qualifications of the lab techs. The tests will only be as good as those doing the work—did the lab technicians correctly follow accepted procedures?
• Qualifications of the people doing the analysis. To establish the reliability of any test, the proponent should offer the testimony of a qualified molecular biologist and a population geneticist. Ideally, these witnesses should be from the academic community, not from the private lab that did the work, because those working in the private sector arguably have a financial interest in the acceptance of their work.

Challenging DNA Evidence in Court

While DNA evidence is capable of supplying reliable evidence, it is not bomb-proof. In light of the factors affecting its integrity, lawyers who challenge its conclusions commonly focus on the following:

• Collection. How was the sample collected? Did the officer use protective gloves, refrain from using plastic bags (which contribute to humidity), isolate the sample from other biological items?
• Transportation. How was the specimen preserved?
• Isolation. In the lab, were proper isolation protocols followed?
• Testing techniques, qualification of the technicians and experts. What methods were used to test the sample? How familiar with accepted techniques were the persons doing the testing? Were the analysts true experts, and unbiased?
• Database sufficiency. Was the database that was used to generate the statistical probability of a coincidental match sufficiently broad?

When Can the Police Collect DNA Evidence?

Until relatively recently, state and federal courts were at odds over the ability of the police to obtain DNA samples from arrested people. Did they need a warrant? And when arrestees were later not prosecuted, or found not guilty, did the state have to destroy its record of their DNA?

In 2013 the U.S. Supreme Court answered at least one of those questions, as far as the Fourth Amendment to the U.S. Constitution is concerned: Police have the right to conduct a buccal (cheek) swab to recover saliva, from which to extract DNA, just as they have a right to obtain fingerprints. However, the Court seemed to say that this right extends only to arrests for serious offenses (felonies), not misdemeanors. And the statute involved in the case allowed for expunging the record if the arrestee was not convicted. Must all state laws have the same expungement provision? (Maryland v. King, 133 S.Ct. 1958 (2013).)

It’s important to note that the King case was decided on federal Constitutional grounds. States may offer defendants more protections than announced in King. Moreover, because the King case was not clear on two key points—which arrestees may be swabbed, and can the state keep the results even when the arrestee is not convicted—states have been free to fashion their own rules on these subjects.

In California, for example, police may obtain DNA samples from all persons, including juveniles, arrested for specified felonies and misdemeanors. (Cal. Pen. Code Section 295 and following.) However, California provides certain defendants—those whose convictions have been reversed and dismissed, who have been found not guilty, or who have been adjudged factually innocent—the ability to expunge their DNA records from the state database. (Cal. Pen. Code Section 299.) These defendants must have no past or present pending charge which would qualify them for inclusion within the database. Interestingly, California’s scheme was held unconstitutional by the appellate court in People v. Buza, 231 Cal. App.4^th 1446 (2014). That case is now before the California Supreme Court; if it overturns the appellate court, the statute will remain in force.