Forensic genetics: how biological samples can be used in solving crimes

  • Share This
Ricky Joseph

Forensic science as a whole reached its climax in the 80's, when direct DNA analysis - through identification techniques - started to be considered fundamental in helping to solve crimes. Since then, many biological traces have entered the list of identifiers: what used to be collected only in blood, today can be seen in several samples: fingernails, saliva, urine,teeth, semen, hair, bones, etc. It is the careful analysis of these materials, respecting what we call "chain of custody", that helps to identify criminals and victims and unravel the facts a little better, since DNA has a high chemical stability even after long periods of time.

The branch of genetics that works with these factors is the forensic genetics which makes use of analysis and sequencing technologies.


In England, in 1988, the rape and murder of two young women (Lynda Mann and Dawn Ashcroft) were the first criminal cases solved with the help of DNA tests. Semen samples were collected (left on the body of each victim) and, believe me, this prevented the wrong individual from being penalized: Richard Buckland, who confessed to both crimes, but was not the perpetrator.

It turns out that a medical geneticist named Alex Jeffreys lived in the same community where the crimes were committed. Years earlier, in 1985, Jeffreys published an article in Nature magazine, commenting on methods that allowed a person to be identified in a totally effective way. The police, in turn, decided to propose to Alec to perform DNA tests from semen found in the victims and comparetheir results to Buckland's own DNA. The tests were done, the semen on both victims belonged to the same man, however they were not Richard Buckland's.

The task involved the analysis of 3,600 samples, belonging to different men who lived in the county. None of them were a match and the case was discredited and consequently shelved. The resolution was made in an unexpected way: in 1988, the police received an anonymous call stating that a bakery worker (Ian Kelly) had donated blood in place of a colleague (Colin Pickford). GoingBy tracking down Colin and extracting a sample of his blood, the police and Alex were able to confirm Colin as the rapist of the two teenage girls, making him the first subject to be convicted thanks to a DNA test.


For many years in history, justice was done through "obvious" material evidence (flagrant) or witnesses. In cases whose authorship was doubtful, or even in which the crime did not have a spontaneous and immediately visible materiality, the possibilities of corruption were numerous before a jury that was not always fair or impartial. If today we still discover cases where innocent people wereaccused and imprisoned as guilty, we cannot count the times with which this error has been reproduced in past centuries.

One of the first instances where the scientific method helped solve a crime occurred in China in 1247. A lawyer of the time named Sung T'su (not to be confused with Sun Tzu) solved a murder committed with a rice harvesting sickle, a tool in common use at the time. T'su publicly arranged all the sickles of all the rice workers out in the open and then observed whether anyThe owner of the tool was condemned and there was born the forensic entomology (study of insects).

The name "forensic" is given to the fields of science that are dedicated to solving crimes and legal matters. In addition to forensic genetics, we have forensic entomology (already mentioned), forensic ballistics, forensic toxicology, forensic medicine, forensic psychology, among others. Each area uses its knowledge in order to unveil the materiality of the fact. That said, it should be noted that geneticsforensics can be applied not only in humans, but in the identification or individualization of animals in general, plants and microorganisms.


Have you watched CSI series and think it's that easy? It's not that simple. DNA is delicate and when not stored under appropriate conditions, it will degrade until there is not enough genetic material left for a useful analysis. It is also worth noting that forensic samples are usually not collected or found under ideal conditions, so they are easily exposed to contamination, a factor thatThis is not only because of the necessary isolation of genetic material, but also because there may be material from the individuals who handled the sample. We will look at some types:


Blood is one of the most commonly found biological traces, especially at crime scenes. These samples can not only identify those present at the scene, but also show if they were under the influence of alcohol, drugs, poisoning or numbing.

There is also reconstructive forensic hematology, which is dedicated to performing bloodstain analysis at the crime scene to reconstruct the actions of the perpetrator and victim.


They are usually a good source of genetic material, since in case they have been used for sexual intercourse, they will contain biological fluids from both parties involved in the act (semen, vaginal secretion or even blood). The success rate reaches 81%*.

Condoms can be used as forensic genetic samples in cases of suspected sexual violence.


It is one of the samples with the lowest risk of contamination, besides the fact that it can be collected in a very simple way and does not require great storage specificities.


The samples can only be used if there is body fluid present on the garment. Underwear can also be used as forensic genetic samples in cases of suspected sexual assault.


They are more useful than hair for extracting genetic material, since they preserve a greater amount of DNA even when cut (compared to hair strands) They have a success rate of approximately 90%.


Despite the popularity, hair strands need to be with the root in order to have genetic material for analysis, that is, they need to have been pulled from the scalp. Despite the ease of collection, the success rate is 75%.


Although not an easy undertaking, genetic material can still be extracted years after the autopsy was performed. Samples can be used as a last resort, as the process is costly and requires careful preparation and handling.

Other samples

Although I cannot say immediately whether the sample will be successful in the analysis, any material that has come in contact with some biological fluid is a potential source of DNA.


Besides the collection and handling being activities that must be performed by qualified individuals in forensic sciences, there are strict standards to be followed to maintain the integrity of the samples. The method depends on the location and condition of the biological material, however, it is essential that each item is handled and packaged separately, identified and properly sealed. The conditionof each material should be documented before being taken to the laboratory, where the samples will be stored properly according to type and condition.


Medeiros RJ. A Genética na Prova Penal. Ed. Pilares; 2009.

Garrido RG. Evolution of Human Identification Processes: from anthropometric characteristics to DNA. Genética na Escola 2009; 5: 38-40.

* The statistics of success of the samples were taken by the laboratory "DNA Barato", you can check on the site: //

Ricky Joseph is a seeker of knowledge. He firmly believes that through understanding the world around us, we can work to better ourselves and our society as a whole. As such, he has made it his life's mission to learn as much as he can about the world and its inhabitants. Joseph has worked in many different fields, all with the aim of furthering his knowledge. He has been a teacher, a soldier, and a businessman - but his true passion lies in research. He currently works as a research scientist for a major pharmaceutical company, where he is dedicated to finding new treatments for diseases that have long been considered incurable. Through diligence and hard work, Ricky Joseph has become one of the foremost experts on pharmacology and medicinal chemistry in the world. His name is known by scientists everywhere, and his work continues to improve the lives of millions.