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Are DNA tests infallible?

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Forensics: exact science or dramatised fiction? #11

Thanks to TV serials like the ‘CSI: Crime Scene Investigation in the USA’ and ‘CID’ many people in India- police officers, forensic personnel and the legal fraternity have come to believe that the so-called ‘DNA fingerprinting tests’ are infallible. No questions are ever asked and sceptics are sought to be silenced by inanities like, “Do not ask, it’s DNA, stupid!”

Like all other forensic techniques, this too is far from the truth even though this is relatively the best of all the forensic techniques. Still, one must always keep in mind that not every case lends itself to DNA analysis. The American Society of Crime Laboratory Directors has concluded that DNA evidence comprises only about 10 per cent of casework and is not always relevant to a particular case. For example, it failed in the most publicized case of the 20th century, the murder case involving the football star O.J. Simpson.

The basics of the technique

To move further let’s first understand what DNA fingerprinting exactly is. In the fewest and simplest words, more than 99.9 % of everyone’s genome is exactly alike (100% if you happen to be identical twins). However, the tiny bit that is not identical is what makes you physically and mentally different from anybody else. The 99.9% genome makes us simply human animals or homo sapiens with the same biology and physiology. Our individuality as different persons comes from the 0.1% DNA.

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If you want to identify people based on their DNA, you will have to focus on that tiny part of DNA, which is distinct in every person. The technique is popularly known as DNA fingerprinting. DNA fingerprinting uses chemicals to separate strands of DNA and reveal the unique parts of your genome. The technique of DNA fingerprinting was developed in 1984 by British scientist Alec Jeffreys. Now, there are different technologies like PCR, RFLP, and STR DNA (Short Tandem Repeat DNA) etc.

The key to knowing a person’s DNA profile is to know which alleles (a variant form of a gene) they have. When an adequate quantity of DNA is present in the evidence, the ‘peaks’, the positions of which represent which alleles are present, are rather prominent.

Scientific limitations of the technique

Degraded or little DNA: The technique becomes inaccurate and gives misleading results if the DNA from the crime scene is in a very small quantity, poorly preserved, or highly degraded. It means that only a partial DNA profile can be obtained. When fewer than 13 STR loci are examined, the overall genotype frequency is higher, therefore making the probability of a random match higher as well. Such situations call for a special technique called mtDNA (Mitochondrial DNA) analysis. Other techniques would give wrong results.

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Female DNA swamping male DNA: The results are misleading if there is a large amount of female DNA, which could swamp any male cells present. Typical examples include sexual assault without ejaculation, sexual assault by a vasectomized male, male DNA under the fingernails of a victim, male ‘touch’ DNA on the skin, and the clothing or belongings of a female victim. In these cases, the absence of sperm would mean that only a small amount of DNA would be present on vaginal swabs after ejaculation and the female’s DNA would swamp these. Such situations call for a special technique called Y-STR analysis. Other techniques would give wrong results.

Secondary transfer of DNA: Secondary transfer occurs when DNA deposited on a surface transfers to a second surface. Unless courts are aware of this possibility, this could potentially implicate an innocent.

Human Errors in analysing DNA evidence: Human errors can play havoc in a complicated analysis like DNA. As Deborah Rogers et al inform, University of California criminology professor, William Thompson has concluded, “I found consistent distortions of the statistical certainty of the DNA evidence. I found instances that looked like fudging of results, to fit the prosecution’s theory of the case, and I found that the lab consistently failed to use appropriate scientific procedures.”

Possibility of malicious ‘planting’ of DNA: Tania Simoncelli, a Science and Technology Fellow for the American Civil Liberties Union, has pointed out the problem of ‘crime framing’, that is, planting DNA evidence to frame someone else for the crime. Anthony Turner, a convicted rapist, smuggled a sample of his semen out of prison, concealed in a ketchup packet.  Subsequently, his relatives paid a woman to use the sperm to complain of a fake rape. The idea was that if the vaginal swab matched with Turner’s DNA in the DNA database, it could be argued that a man inside the prison could not have committed rape and hence there must be something indeed wrong with the DNA testing. It would automatically mean that his earlier conviction based on DNA testing also becomes doubtful. 

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The technique fails completely in the case of a mixture of DNA

In the early euphoria of DNA fingerprinting, it did not occur to police officers and the forensic community that very many crime scenes present mixtures of DNA. In crimes, such as gang rapes, the vaginal swab is very likely to contain a mixture of the DNAs of the assailants. As Rich Press explains for the US government’s National Institute of Standards and Technology, high sensitivity of the technique becomes a liability then.

In the case of a mixture, one has to deal with at least two uncertainties. As Rich Press explains, when the amount of DNA is very low, the peaks can be very small. Some peaks can be so small that they disappear entirely (in other words, they ‘drop out of the profile). In addition, small blips in the data can be mistaken for real peaks (they ‘drop in’ to the profile). Many of these effects are random, and they can make it difficult to interpret the evidence.

Then there is a fundamental problem—whose peak is it anyway? There will be alleles from all the contributors. Think of it as an alphabet soup with each letter representing a different type of allele.

Suppose your suspect is named RADARENDRA GOBHI. Suppose you analyse the soup and find that all the letters in the suspect’s name are present. Does that mean that someone named RADARENDRA GOBHI only contributed to the soup?

No. For example, suppose instead of RADARENDRA GOBHI, you had two persons with names say, DARPOK HAI and FEKU GOBEN who would have contributed their DNAs. In that situation also, the soup would have all the letters needed to spell RADARENDRA GOBHI, even though no person with that name would have contributed to the soup.

Just because a person’s alleles appear in a mixture does not mean that person would have necessarily contributed to it. The alleles might very well have come from some combination of other people who, between them, would have all the allele types in the suspect’s profile.

We spoke of small peaks possibly ‘dropping out’ when the DNA is insufficient. In terms of the soup analogy, it would be tantamount to a letter becoming so small that it disappears. Suppose the letter N in the example given above drops out. Now, this means that the suspect’s name would become RADAREDRA GOBHI. In that situation, a much larger number of persons could theoretically contribute to the soup. For example, you could have three persons DARPOK HAI, FEKU and GOBAR or many other possibilities.

This means that the evidence would become weak. It will be difficult to state whether a suspect might have contributed to a mixture or not. Instead of a simple yes or no, scientifically, we will be obliged to speak in terms of probabilities.

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For dealing with DNA mixtures, they have developed special software. Probabilistic Genotyping Software (PGS) uses statistical and biological models to calculate probabilities. After computing these probabilities, the software produces a number called a likelihood ratio. That number is the software’s estimate of how much more or less likely it is to see that mixture if the suspect did contribute to it than if he did not. It still leaves considerable scope for error. In any case, it is a difficult thing to interpret and, realistically speaking, few jurists can really comprehend that or translate it into a verdict.

It should also be kept in mind that the type of software used, how the software is configured, and which models the software runs can all affect the results. This means different labs might produce different results when interpreting the same evidence. Sometimes those differences can be large enough to call into question the reproducibility of the results. This highlights the fact that every scientific method has its limits, and some mixtures will be too complex to reliably interpret even with PGS. Currently, there is no consensus on how to identify those limits.

Then there is another complication. While PGS can try to tell you who might have contributed DNA to a mixture, it cannot tell you how or when their DNA got there. For example, if someone had entered a house after breaking the glass window and if he cut himself in the process, his blood would be there and DNA analysis could be reasonably attempted. However, suppose he made a duplicate key and entered through the door, the doorknob would contain a mixture of DNA from all those people who might have used the knob recently. This is another example of the secondary transfer of DNA.

Get real, not euphoric

In view of the above, we must accept that DNA fingerprinting too has its inherent limitations and the invocation of the very word DNA should not spur the public, the media, the cops, the lawyers and the courts into presuming that the final word has been spoken.

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Dr N C Asthana IPS (Retd)
Dr N C Asthana IPS (Retd)
Dr. N. C. Asthana, IPS (Retd) is a former DGP of Kerala and ADG BSF/CRPF. Of the 56 books that he has authored, 20 are on terrorism, counter-terrorism, defense, strategic studies, military science, and internal security, etc. They have been reviewed at very high levels in the world and are regularly cited for authority in the research works at some of the most prestigious professional institutions of the world such as the US Army Command & General Staff College and Frunze Military Academy, Russia. The views expressed are his own.


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