Changing the role DNA plays in the justice system

DNA is a genetic code made up of 4 letters (A, T, G, C), called DNA bases, that are interpreted by our cells to make the molecules and structures that allow our bodies to function. Regions of DNA that encode molecules known as “proteins” are called genes. The unique code in every person results in physical differences—such as brown or blonde hair and blue or brown eyes—between individuals. It can also be used for identification purposes. Although the vast majority of DNA (99.9% on average) between two individual humans is the same, scientists have characterized regions of DNA that are different between people who are not closely related.

The most commonly used method of genetic testing in forensics looks at these variable sections of DNA. Forensic labs look at 20 DNA regions that vary between individuals, called short tandem repeats (STRs), to create a DNA “fingerprint” These STRs are located in stretches of DNA between gene-coding regions and consist of short DNA sequences (e.g. “TATT”) that are repeated different numbers of times in different people. For example, in person A, the stretch of DNA may be “TATTTATTTATT” (three repeats), but in person B, the same region of DNA may be “TATTTATTTATTTATTTATT” (five repeats). Labs can then compare the number of repeats at each of these STRs to a sample taken from a crime scene and calculate the probability that the DNA from a suspect matches that sample. The chance that two people who aren’t closely related have the same DNA profile is 1 in 1,000,000,000,000,000,000.