by Mary May
figures by Rebecca Clements

As anyone who has watched an episode of CSI can attest, catching a killer is only a DNA sample away. Due to advances in DNA testing technology and its omnipresence in forensics (as portrayed on TV and in pop culture), the public has come to expect and trust genetic testing as evidence in criminal trials. As these methods become more sophisticated and more difficult for the general public to understand, the proper role for DNA testing in forensic science remains undefined. It is key that the public understands the potential—as well as the limitations—of DNA-based evidence to best determine the role that genetic testing should play in the justice system.

DNA forensics: Creating a DNA fingerprint

Our 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” (Figure 1). 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.

Figure 1: Creating a DNA “fingerprint.” DNA profiles made from STR analysis are like a fingerprint or very long social security number. We can use them to calculate the statistical likelihood that different DNA samples came from the same person. Because each person inherits two copies of each gene in a cell (one from their mother and one from their father), they can have two numbers of repeats at each STR. The chances that two unrelated people have the same number of repeats at all 20 STRs is extraordinarily small.

Progress in DNA sequencing technology

As technology has progressed, scientists have been able to create these DNA fingerprints with much smaller DNA samples, meaning that a suspect can be identified from a drop of blood instead of a pint. One new technological development, Next Generation Sequencing (NGS), sequences, or reads through, many small fragments of DNA at the same time, giving results much more quickly and at a lower cost than older methods. Accordingly, the number of regions used in STR analysis was increased from 13 to 20 in 2017, increasing the accuracy of DNA testing.

Scientists have also developed methods to analyze mixtures of DNA samples, as might occur when DNA is collected from a rape victim. For example, sophisticated software uses probabilistic genotype matching to determine the chances that two samples come from the same person. Analysts use this software to calculate the likelihood ratio, which measures how much more a suspect matches the data than a random person would.

The future of genetics in forensics: Using DNA to predict appearance

Scientists have developed models that can predict either blue or brown eyes over 90% of the time and brown, red, or black hair 80% of the time by looking at the variation in different genes between individuals. Scientists are now working on models that can predict complicated facial features which may be affected by hundreds of genes (Figure 2).

Figure 2: Translating DNA to appearance. Small differences in many genes interact to give rise to differences in physical appearance. Companies such as Parabon Nanolabs and Identitas hope to exploit our growing knowledge of genetics to create portraits of victims and suspects from DNA samples alone.

In the future, we may go much further than just comparing evidence from a crime scene to a known suspect. Instead, we may use DNA from crime scenes to create descriptions of potential suspects or unidentified victims from scratch via a method called DNA phenotyping. In a racially biased criminal justice system, this technique has the potential to help reduce discrimination by preventing police from targeting the wrong people due to racial bias. Police have already used the technique to help identify victims from cold cases.

Although the technology supporting forensic genetics is improving, the science behind DNA phenotyping is still controversial, with many scientists saying companies such as Parabon Nanolabs and Identitas promise far more than they can deliver. Although police may use these companies to open new leads and identify suspects, evidence from DNA phenotyping is not currently permitted in courts to convict defendants. As technology improves and police increasingly use DNA phenotyping, it’s important to consider what role it should eventually play in the justice system.

The limitations of DNA testing and its role in the justice system

The complexity of the statistical methods used to analyze DNA samples and draw conclusions poses a challenge to attorneys and judges, who must understand how they work to reliably assess their validity in court. DNA samples are fragile and can degrade over time, which can lead to errors during the sequencing process, especially if the amount of sample material is small. Inconsistent methods for interpreting DNA sequencing data, especially that of DNA mixtures, leads to inconsistent verdicts on the identities of DNA donors. Complicating matters further, incentives for labs to find matches could lead to bias on the part of the forensics labs performing genetic testing. It is essential both that those evaluating DNA evidence understand these caveats and that the government continues to promote the improvement of these technologies to reduce uncertainty and error in the analysis of DNA evidence.

DNA testing in forensics science has proven to be a powerful tool for both catching criminals and exonerating innocent people. As technology improves and the applications of DNA sequencing expand, we must ensure that the science underlying the analysis used to make decisions in court remains transparent and validated by the broader scientific community. Soon, we may witness DNA testing in the forensics lab catch up to the science fiction of television.

Mary May is a fourth-year Ph.D. student in the Chemical Biology program at Harvard University.

Rebecca Clements is a third-year Ph.D. candidate in the Biological and Biomedical Sciences program at Harvard University. You can find her on Twitter as @clements_becca.

Further Information:

  • To read an in depth article about advances in genetic testing technology as well as how genetic testing can lead to false convictions, read this article from The Atlantic.
  • To learn about the basics of genetic testing and the ethics of using a database of convicted felons’ DNA profiles, see this educational article from Nature.
  • To read about using DNA to predict physical features of individuals, read this article in Cell, and to learn about how police are using this information, read this article from The New York Times.

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