UNRELIABLE EVIDENCE

Indeed, it was “junk science” that convicted Roy Brown. The only physical evidence linking Brown to the case was his teeth. A dentist hired by the prosecution testified that the bite marks on Kulakowski’s body matched Brown’s teeth. But as the defense pointed out, the bite marks came from someone with six upper teeth—Brown had only four. The prosecution’s witness argued that Brown could have twisted Kulakowski’s skin while biting her and filled in the gaps—an argument that ultimately proved convincing to the jury.

Bite-mark analysis is a particularly troubling form of evidence. No widely accepted rules or standards govern its use, and no government or outside scientific commission has ever validated its claims. In fact, studies show error rates—the rate at which experts have falsely identified bite marks as belonging to a particular person—as high as 91%.

Hair analysis, another common type of evidence, can be equally troublesome. In dozens of cases, Innocence Project lawyers found that forensic scientists had testified that hairs from crime scenes matched the accused, explains Neufeld. But when scientists subsequently tested the DNA inside the follicle cells from those hairs, the DNA didn’t match.

The problem is that hair analyses, performed under a microscope, can reveal only certain characteristics: it can distinguish whether hair is human or not, show a person’s ancestry (because of ethnic differences in hair texture), whether the hair has been dyed, cut in a certain way, or pulled out, and where on the body it came from. Hair samples can exclude a suspect, but not positively identify one.

By contrast, each person’s DNA is unique. To understand how DNA varies from person to person, consider its structure. DNA is made up of two strands of subunits linked together in long chains. Each subunit—called a nucleotide—has three parts: a sugar, a phosphate, and a base. The phosphate group of one nucleotide binds to the sugar group of the next nucleotide to form a chain of interlinked nucleotides. The two strands of linked nucleotides pair up and twist around each other to form a spiral-shaped double helix. The sugars and phosphates form the outside “backbone” of the helix and the bases form the internal “rungs,” like steps on a twisting ladder. The bases are held together by hydrogen bonds.

The nucleotide rungs, made up of the bases, are most useful in DNA profiling. There are four different possible nucleotide bases: adenine (A), thymine (T), guanine (G), and cytosine (c). In DNA, these four nucleotide bases are repeated over and over, billions of times, in different orders along a DNA strand. The order of nucleotide bases is a key form of genetic information in cells—it provides the instructions for making proteins (see Chapter 9). To identify perpetrators, forensic scientists examine the specific sequence of nucleotide bases along one strand of a person’s DNA—the precise order of As, Ts, Gs, and Cs. With the exception of identical twins, no two people share exactly the same order of DNA nucleotides (INFOGRAPHIC 7.2).