A genetics expert who visited Kathleen Folbigg in jail to get her medical history has told an inquiry a rare genetic mutation may have led to the deaths of two of her four children.
A second judicial inquiry is underway into the convictions of Folbigg, 55, for killing her children, Sara, Laura, Caleb and Patrick, who were all under the age of two.
They died on separate occasions in the NSW Hunter between 1989 and 1999.
The Crown alleged at trial in the NSW Supreme Court that the children were smothered by their mother.
Folbigg maintains her children died of natural causes and the discovery of a rare gene mutation in her two daughters that causes sudden death, has cast doubt on her convictions.
The mutation has not been found in her two sons.
After contributing to research on the rare mutation, CALM2G114R, Todor Arsov, a genetics expert of 20-years experience met with Folbigg in jail in October 2018.
He recorded her family and genetic history, took a mouth swab for genomic analysis and wrote down that she had a history of fainting that could indicate a cardiac condition.
Folbigg told Professor Arsov she fainted at a swimming carnival when she was 11 or 12 years old after feeling unwell, but he could not recall if she said she fainted in or out of the water.
"She woke up on the side of the pool is what I remember," he said.
"I had to be careful, it was somebody going through loss and grieving, and to me at the time the exact circumstances were not important."
Counsel Assisting Sophie Callan SC, asked the witness if he knew Folbigg before visiting her in jail.
"Have you ever spoken to her aside from this visit?" Ms Callan asked.
"No, but I went back again to take samples," he replied.
Professor Arsov said he thought the gene mutation was a "reasonable explanation" for the deaths of Sarah and Laura Folbigg.
Retired Chief Justice, Tom Bathurst, who is presiding over the inquiry, questioned Professor Arsov about his evidence.
"You can't specify these children died from any particular condition," Mr Bathurst said.
"We can talk about probabilities or likelihoods," the witness replied.
Ms Callan asked the witness if he viewed the Folbigg family as "tragically unlucky".
"I think so," he replied.
Folbigg, who has been referred to as the country's worst female serial killer, has five years left of a 25-year sentence.
The inquiry continues.
