at The University of Hong Kong have been helping to solve crime
scene complications where potential evidence is stained with DNA
from more than one person.
the probability of a match between mixed DNA stains found at a crime
scene and samples taken from a suspect later found to be innocent
has been described as unlikely or very unlikely.
of a mixed DNA stain from a crime scene with samples from the
victim and suspect for comparison (above right) Prof Fung with
DNA model (above left)
law enforcers can describe probability as a statistical value thanks
to a research project headed by Principal Investigator, Prof Tony
Wing-kam Fung. For a positive identification, forensic investigations
involving DNA often need at least nine matches at sites, or loci,
identified at specific points across the 23 pairs of chromosomes
found in humans.
Fung drew data from DNA profiling systems used by police in Hong
Kong, Beijing and Taiwan to compare the frequencies of allele, or
distinct types of DNA, at each of these loci.
found the frequencies were almost the same in all three cities,
he said. All loci are non-coding which means they do
not describe the colour of skin, hair, eyes, or ethnic orientation.
Prof Fung went on to verify the extent that the loci were independent.
He said: The nine loci used for identification cannot be 100
per cent unrelated because human beings are not all mated randomly.
wanted to find out which sites were the least related. The more
unrelated the sites are, the easier the calculation. With
mixed DNA stains from a crime scene, samples found by forensic detectives
may include DNA from both the victim and perpetrator.
in principle we are all unique, except for identical twins, the
fragment length of the loci of various individuals should be different,
said Prof Fung.
mixed stains, we can identify the DNA fragments contributed by the
victim which leaves the remaining DNA fragments for comparison with
the suspect. Prof Fungs research has developed the statistical
theories and calculations to help in the analysis of mixed stains.
theorems he obtained can be used in cases where more than 10 people
are involved, as well as for multi-ethnic groups and relatives of
suspects where DNA similarities may exist.
of the principles are being used in paternity testing and is the
basis of a computer application, EasyDNA, developed by Dr Fung.
To date, the software has been used by laboratories in Asia, Europe,
North America and Latin America.
Tony Wing-kam Fung