Genetic testing plays crucial role
DNA science has emerged as a crucial tool for investigators trying to solve Leung Shing-yan's killing.
Forensic experts from the Hong Kong government laboratory carried out DNA profiling on a mask found beside the constable's body on the Shek Wai Kok estate on March 14, 2001, in the hope of finding clues to its wearer.
DNA stands for deoxyribonucleic acid, the nucleic acid which contains the genetic instructions for the development and functioning of living organisms. It is found in every cell of the body, and the genetic coding is the same for all cells.
Henry Cheung Kam-yin, a retired forensic evidence officer, testified in the Coroner's Court yesterday that DNA could be obtained from blood, hair, skin or saliva.
He said the purpose of DNA testing, or genetic fingerprinting, was to compare the DNA sample obtained from the evidence with that obtained from a suspect.
'We use a machine called applied biosystems to measure the length of a DNA [sample],' he said. 'The length can tell the characteristics of DNA.'
Dr Cheung said once the genetic characteristics had been established, a forensics expert could generate a probability ratio - called random match probability - to illustrate the level of likelihood that a DNA sample came from a suspect.
For example, the DNA obtained from Leung's blood showed that there was only a one in 160 billion chance of another Hong Kong Chinese sharing his exact DNA characteristics.
Apart from identifying suspects, DNA testing is also used to clear convicted prisoners. In the US, the Death Penalty Information Centre reports 14 death-row inmates have been exonerated with the help of DNA evidence. A federal law passed in 2004 encourages post-trial DNA testing and provides US states with funding for tests.