Crime Evidence

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Genetic Engineering to Know the Facts

Over the last few decades, and thanks to the rising surge in popularity of movies, series, and books based on crime fiction and journalistic reporting, we have become more and more acquainted with the sophisticated forensic procedures used by crime scene investigators and what goes on in forensic labs. 

Today we know that DNA analysis is useful in identifying criminals who have made a mistake by unwillingly leaving a sample of their body (skin, hair, saliva, etc.) at the scene of a crime. In fact, we have seen this so many times on the American crime series CSI that, if we were to steal a painting from the Louvre, we know we would have to take the proper precautions to ensure that we didn’t leave any DNA evidence behind.

🧬 DNA = ID Card

Each of us is unique. 

This is also the case at the genetic level since, except in the case of monozygotic twins, each one of us has a unique DNA. Only 0.1% of our genome makes us unique, but since the genome is so long (3000 million characters), 0.1% means a lot of difference!

Our DNA is unique because there is an infinite number of possible combinations when the mother’s DNA is mixed with that of the father. It is impossible for exactly the same sequence to be replicated. In this way, our DNA is a non-transferable personal identification document that is found in each and every one of our cells, and we can unintentionally and unknowingly leave traces of ourselves in many places. 

What Is the DNA Identification Test?

DNA is our personal identification card, but it contains more information than an encyclopedia. Thankfully, we don’t have to read all its content to determine who it belongs to as this would take us years. We only need to read a combination of very short fragments to be able to link it to an individual. 

This is particularly easy when we have suspects to compare samples to. It could be a criminal we want to place at a crime scene, or an alleged father we want to identify as the parent of a child. Both qualify as “suspects.” If we compare the DNA fragments (genetic markers) of the reference sample to the sample being analyzed and have a match, this pretty much tells us that the suspect is the culprit (although it may not be 100% accurate).

However, before being able to study in detail a segment of DNA, especially when the sample is very small and contains little DNA (hair, eyelashes, etc.), the segment must be amplified. This means that we need to obtain multiple copies of the segment to be able to sequence (read) the sample. But how can we do that? Well, using a method the lab technicians in CSI know all too well: the Polymerase Chain Reaction, or PCR.

Hair, saliva, dead skin, or eyelashes… we leave our very own “business” card everywhere we go.

🕵️ At the Service of Criminology

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DNA analysis was first used to solve a crime in 1986: the murder of two girls in Leicestershire, England. 

Since then, exponential progress in the field of basic sciences has led to a truly scientific and technological revolution in applied sciences.

The techniques used by forensics have improved a lot since. For example, one such technique is PCR amplification, which allows work to be done on very small segments of DNA. Cases that had not been solved due to lack of evidence are being reopened and investigated using these modern forensic techniques. Some of these so-called “cold cases” date back to the 1950s or even earlier.

⚠️ Attention! This Technique Is Not Infallible

Today, technological advance in forensic science allows the color of a suspect’s hair to be identified, and it will soon be possible to reconstruct a person’s face. But these new technologies require new procedures and protocols to ensure that research is carried out under optimal conditions.

For DNA to be extracted from small and fragile biological evidence, the processes of collecting, storing, and transporting samples must be handled very carefully. If the medical examiner is not aware of this, the evidence may be destroyed or compromised, and any investigation that may reveal information about a crime will not be able to provide evidential value.

Genetic tests are never one hundred per cent accurate, so they should not be treated as foolproof evidence that the suspect of a crime is guilty. Genetic testing is for sure a useful tool, but it must not be taken as definitive proof. Genetic testing is for sure a useful tool, but the result of a DNA test is not a conclusive piece of evidence. It is error-prone and its value is measured in terms of probabilities.

Predicting Criminals’ Surnames

That DNA analysis is also very useful in running paternity tests is a well-known fact. In general, DNA analysis can determine blood relations between people, since close relatives share many fragments of DNA and are therefore physically and psychologically similar.

Researchers in the UK are developing a technique that will link DNA evidence from a crime scene to the surname of the criminal. 

Surnames, like the Y chromosome, have been passed down through history from fathers, and not mothers, to sons. Therefore, males sharing the same surname have a common Y chromosome lineage. The technique consists in comparing the Y chromosomes of pairs of men with the same surname and determining the characteristics they all share. This allows to link certain characteristics of the Y chromosome to a given surname. 

Not surprisingly, many obstacles get in the way of the relationship between genetics and onomastics: adoptions, spousal infidelities, name changes, and biologically unrelated people using the same surname are but a few examples. So, in the UK, common surnames such as Smith, Jones, and Taylor cannot be used for this type of DNA analysis, which means that only 40% of British men sharing the same British surname are eligible for the study.

🧬Ancient DNA

Paternity tests are relatively common today. But what if we wanted to know if a person who lived in the past was a relative of ours? Could we find out? 

The main constraint is the difficulty in obtaining ancient DNA. Extracting good quality DNA from two living individuals is relatively easy, but things take a different turn when we want to use old samples which have become corrupted, as is the case sometimes with corpses. And we’re not even talking about the DNA of prehistoric ancestors! That’s why we need to develop reliable techniques for the extraction and analysis of ancient DNA.

Ancient DNA is usually found in the bones and teeth of corpses. However, it is very difficult to extract it because it is preserved in poor conditions. If we are lucky, a softer part of the body may still be preserved. We could then access a greater amount of DNA and extraction would become less problematic. In general, the older the sample, the harder it is to recover DNA successfully, although it ultimately depends on the preservation conditions.

The best option for studying old samples is mitochondrial DNA.

🦕 Is It Possible to Create a Real-Life Jurassic Park?

So, can we get DNA from old samples? As old as, say, a dinosaur? Can we clone a Tyrannosaurus rex like we saw in Jurassic Park? For now, this idea is unrealizable. Unfortunately, the samples we have, which are more than 65 million years old, are not well enough preserved to even read a single dinosaur gene. And of course, it goes without saying that the chances of finding a full dinosaur genome are rather slim.

However, we’ve been able to recover and sequence the complete genome of a mammoth. Catalan researcher Carles Lalueza-Fox, a renowned biologist working on ancient DNA, was able to successfully extract DNA from mammoth remains found in Siberia that had been preserved in ice for 43,000 years. Through careful analysis of the fragments obtained it was possible to identify a key gene in mammalian hair pigmentation, and we now know the fur color of those behemoths. Interestingly, the creators of the animated movie Ice Age 2 portrayed some mammoths as having dark fur, while others had lighter or reddish fur. And guess what? They got it right!

Analysis of ancient mammoth DNA has allowed us to know what color those animals were.

🧬 Two Types of DNA

Broadly speaking, nuclear DNA determines our traits and is found in the cell’s nucleus.

Mitochondrial DNA is found inside mitochondria, cell organelles that produce energy from the oxygen we breathe. Mitochondria have their own DNA and multiply inside the cells because millions of years ago, before animal-type cells existed, they were free-living bacteria. All our cells have mitochondria (apart from red blood cells). But unlike nuclear DNA, which we inherit in equal parts from our father and mother, all mitochondria come from the mother. This is explained by the fact that at the time of fertilization only the head of the sperm enters the egg, which contains nuclear DNA. So we know that mitochondria are not passed on through the father’s sperm. All the mitochondria we have in our body are the descendants of those in our mother’s egg.

It is better to use mitochondrial DNA in the study of old samples because it has thousands of copies per cell, and since no genetic information from the father is used in this DNA testing, issues derived from possible illegitimate children are avoided. With the direct type of inheritance, the fragments of DNA from the father and the mother do not get mixed (we call this recombination). Therefore, the mitochondrial DNA that passes down from the mother to the child is very well preserved, and we can use it to identify remains even if the reference sample and the evidence sample are separated by several generations.

🤷‍♂️ Tracing the Genetic Ancestry of Humankind

But where did the first humans appear? How did the colonization of all regions of the world come about? Whom are Europeans more closely related to, Africans or Asians?  

These are some of the questions that DNA sequence analysis of human populations can help us answer. Population genetics is the field of science that looks for answers to these questions. By analyzing fragments of DNA from hundreds of people from various regions of the planet, scientists have succeeded in outlining the family book of the human race.

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A group of researchers proposed in 1987 that the genetic diversity of the mitochondria of all modern humans could be traced back to a single female ancestor that lived in Africa about 200,000 years ago. This theory has come to be known as “Eve’s Mitochondrial Theory.”

Their hypothesis does not defend the idea that back then there was only one woman. Instead, it suggests that of all the women who lived at that time, only one managed to have an uninterrupted maternal line to this day, since at some point in history each of the other women had only male descendants. It provides support for the theory that holds that Homo sapiens originated in Africa and spread throughout the planet just over a hundred thousand years ago. Moreover, most of the genetic evidence we have gathered points in that direction.

The early evolution of modern humans, however, is a very active area of ​​debate and research. There are two opposing theories:

The replacement hypothesis argues that all modern humans descend from an ancestral population of Homo sapiens that appeared in Africa some 200,000 years ago. This population spread across all continents and replaced the other human species that had evolved from a previous expansion of Homo erectus.

The multiregional hypothesis holds that after the first expansion of Homo erectus about 1.5 million years ago, its descendants evolved towards Homo sapiens in several places throughout the world despite the geographical distances separating the various populations. Normally, when a population is isolated from other populations of the same species, it tends to evolve differently than the others. But the multiregional theory argues that the various human populations evolved into a single species thanks to gene flow (that is, the continuous interbreeding events that occurred between individuals from different populations, which led to their homogenization). 

While the debate remains open, the two lines of study are hoping to find conclusive evidence that allows us to accurately trace the origin of humanity using the tools provided by genetics and paleoanthropology. Undoubtedly, this investigative work is as thrilling and exciting as a Conan Doyle novel or the best episodes of The Wire or True Detective.


WRITTEN BY
The ISTF team

IMAGES
(1) Thanx, ©sxc.hu; Steven Foley, ©iStock.com
(3) Scientist with DNA sample / D-Keine, ©iStock.com
(4) CSI, Crime Scene Investigation / © CBS
(5) Police detention / © Michal Zacharzewski
(7) Dinousaurs: Micropachycephalosaurus (left) and Eoraptor (right) / Oriol Massana
(8) Ice Age 2 / © Twentieth Century Fox
(10) Rock art / © Reinhardt Hoft
(11) Homo sapiens / Oriol Massana


 

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