Wynarka child murder: How DNA of victims Khandalyce and Karlie was linked

IT sounds simple: extract DNA, find a match, solve a crime. But ask two of SA’s top experts to explain how murder victims Karlie Pearce-Stevenson and daughter Khandalyce were linked, and you’ll get a detective story of extraordinary complexity.

• MAIN REPORT: Family pleads for aid as police hunt killer
• BREAKTHROUGH: The one phone call they needed
• THE REACTION: Wynarka residents relieved for ‘little angel’
• KEY QUESTION: Why was the missing persons report withdrawn?
• THE HIDING SPOT: Belanglo, the forest of nightmares

It sounds so simple. Extract some DNA, find a match and solve a crime mystery. But when Science Reporter Amelia Broadstock asked two of Adelaide’s top genetic experts to explain how murder victims Karlie Pearce-Stevenson and daughter Khandalyce were identified and linked, she instead found a detective story of extraordinary complexity.

THE EXPERTS: Professor Adrian Linacre, Flinders University, chairman in Forensic DNA Technologies, Flinders University

Associate Professor Jeremy Austin, University of Adelaide, deputy director of the Australian Centre for Ancient DNA

What is DNA?

Prof Adrian Linacre: DNA is an information molecule that encodes all the necessary information to create any living organism. Just like a computer that uses binary codes or 0s or 1s, DNA uses four units, called bases and termed A, C, G and T, to encode all the molecules needed for life.

What does DNA look like?

Linacre: You cannot see DNA unless using an incredibly powerful microscope. But it was worked out in 1953 that the bases are arranged in a particular order to form the famous double helix, with a G facing a C, and an A facing a T. The number of pairings in a human is close to 6.4 billion.

What is the process of identifying a person’s DNA?

Linacre: DNA is only one molecule within a cell and needs to be isolated from all the others. This is not straightforward, but isolation processes rely on DNA having a negative charge and therefore using a positive charge can pull the DNA away from all the other molecules. This takes about three hours to complete from blood and saliva, but DNA in bone is much more difficult as there is a huge amount of calcium and other bone material and very little DNA.

Once isolated, the DNA needs to be quantified to see how much, if anything, was found. Currently the 6.4 billion bases of a human in every cell is too big to analyse routinely, therefore small sections of DNA known to be variable are targeted and a DNA code recorded.

How did scientists link DNA from the little girl found in the suitcase at Wynarka, to her mother’s bones found dumped in Belanglo State Forest in NSW?

Ass Prof Jeremy Austin: We inherit half our DNA from our mother and half from our father – thus the DNA profile of a child will be a 50 per cent match to their mother and a 50 per cent match to their father. The method used by scientists to link the girl to her mother relies on this knowledge – its called familial searching. Essentially we are looking for DNA profiles that are similar to each other, suggesting a familial relationship, brother-sister, mother-child for example. The same methods would have been used to identify parents and children in cases where families were killed in a single disaster, like a plane crash or the Boxing Day tsumani.

Can any part of the body be used for DNA testing?

Linacre: Our DNA is the same in every cell as this came from the fusion of one egg and one sperm. There is DNA in white blood cells, skin cells, such as present in saliva, and indeed in teeth and bone. All the DNA is essentially the same, allowing comparison from a bone sample to a blood sample.

Is the DNA identification process different using older bones?

Austin: Yes the process is slightly different. The DNA in older bones is quite degraded because the DNA strands are broken into shorter and shorter pieces and much of the DNA is completely gone. We need to use special techniques to extract the very small amounts of degraded DNA from the bones. The bones can also be easily contaminated with DNA from other people, or animals, so we need to work in a very clean laboratory to minimise contamination from other sources. The DNA profiling is also much harder as it can be difficult to generate a complete DNA profile from such small quantities of DNA. There are a number of techniques that can be used to help produce a full DNA profile, but these are much more expensive, and take much longer, than normal DNA profiling.

How has technology helped with DNA profiling?

Austin: DNA profiling has changed over the last 10-15 years. We can now profile more DNA markers, which increases the power of identification, by reducing the chances that any two people will have the same DNA profile. DNA testing is much, much more sensitive, so we can get results from less DNA. Also, we now have access to genome sequencing technologies, which allows access to much more DNA information about a person, not just the DNA markers for identification, but also a persons ancestry – are they European, Asian, African or a mixture of more than one – and their hair and eye colour.