How DNA test can establish guilt or innocence

Esther Ng’eno, 80, lived alone, but when neighbours didn’t see her for a few days, they broke into her house and stumbled upon a shocking scene, she had been raped and strangled to death.

There was no witness to the killing, and appeared as another hopeless case to follow up.
How investigators eventually identified the suspect is credited to forensic technology that experts want the Government to adopt to keep criminals on a short leash.

Investigators obtained traces of semen from her clothes. And after DNA tests, they set out to zero in on the suspect.

A young man employed to look after the old woman’s cattle was one of those questioned during the investigations, but he denied being at her home on the fateful night.

And to prove innocence, he unsuspectingly volunteered a sperm sample.

Forensic tests confirmed the investigators’ suspicion. His DNA matched samples collected at the scene. This was one of the cases in Kenya where science has been used to identify individuals in a judicial process.

So how does DNA profiling work?
Dr John Mungai, who is in charge of the Forensic DNA unit at the Government Chemist, explains that it entails collecting DNA-containing material to resolve disputes in criminal and civil cases.

“Every contact leaves a trace,” Dr Mungai says, adding that even relatively minute quantities of DNA at a crime scene can yield sufficient material for analysis that helps detectives prove the culprit’s guilt or establish his innocence.

He notes that DNA short for deoxyribonucleic acid is uniquely found in the cells of all living things and it contains the instructions that an organism needs to develop, survive and reproduce.

He says the six sources of DNA-containing material are soft tissue, blood, semen, saliva, hair roots and dead skin cells are collected at crime scenes and taken through a five-step identification process.

At the 4th African Society of Forensic Medicine conference, Dr Mungai presented a paper titled, ‘Application of DNA in Solving Crime; including Gender Based violence,’ and detailed Kenya’s progress since DNA profiling began in 1997.

He cited poor management of crime scenes, lack of information by victims of sexual violence who fail to appropriately preserve the clothes they had worn when the crime was committed adding that with the right analysis techniques, science and technology has made it easier to establish profiles of individuals.

Its forensics that made the infamous blue Gap dress, worn by Monica Lewinsky during a sexual encounter with Clinton, hit global headlines.

A blood sample was taken from Clinton on August 3, 1998 and on August 17, 1998 the FBI reported its conclusion that Clinton was the source of the semen on the dress “to a reasonable degree of scientific certainty.’

It is on this premise that Chief Government Pathologist Johansen Oduor is optimistic that setting up a DNA database for criminals in order to identify and have a surveillance of repeat offenders is an efficient system to keep off offenders and prevent crime.

“Forensic science is critical to arresting offenders thus enhance an efficient judicial system,” said Dr Oduor and challenged investigating officers to be thorough and detailed in their documentation.

“It is not acceptable that murder suspects get away due to gaps in the documentation of evidence,” he added.

In developed countries, they have moved a step further by looking for suspects based on faces generated by a computer relying solely on DNA found at the scene of the crime.

Dr Mungai explains that the same analysis is used in disputed paternity cases and the tests are extremely sensitive, and can be conducted using samples that would be too small for other blood tests.

“In a paternity test, the mother’s DNA profile is compared with the child’s to find which half was passed on by the mother. The other half of the child’s DNA is then compared with the alleged father’s DNA profile,” Mungai says.

If they don’t match, the ‘father’ is excluded, which means he isn’t the father of that child and if the DNA profiles match, the ‘father’ is included - which means there is a high probability (more than 99 per cent) that he is the father.

Mungai is optimistic that in future, genetic sleuths will work faster in profiling a suspect leading to subsequent arrest and prosecution.

Related Topics

DNA test