When questioned by Farrow's barrister Peter Gower QC about the possibility of "secondary transfer" - meaning that Farrow may have touched a surface and Mrs Yates picked up the DNA from that surface - Mr McKenzie said it would be "very rare" to detect DNA from a secondary transfer.
The court heard a number of swabs were taken from Mrs Yates but the sample from the back of her left hand gave the strongest profile.
In court today, forensic Scientist Christopher McKenzie told the jury:
"We found that it is a billion times more likely to have come from Betty Yates and Stephen Farrow than from Betty Yates and someone other than Stephen Farrow."
Mr McKenzie said he would expect the DNA to only stay on the hands for a relatively short period of time.
"Typically washing is the best way of removing it, or contact with other surfaces," he said.
"You would expect the hands to be regularly washed and in contact with other items."
Mr McKenzie said given the strength of the DNA profile he would expect there to have been either direct physical contact between Mrs Yates and Farrow or for it to have come from a bodily fluid - potentially saliva or sweat.
Stephen Farrow denies her murder.
The trial continues.
Forensic Scientist Christopher McKenzie has been giving evidence in the Betty Yates murder trial.
The court heard that DNA representing a one-in-a-billion match to that of a retired teacher from Worcestershire and the man accused of her murder was found on the back of her hand.
A "full DNA profile" of Stephen Farrow, 48, was found on a swab taken from Betty Yates's left hand after the discovery of her body.
Farrow, of no fixed address, claims he saw Mrs Yates on December 30th but Bristol Crown Court heard the location the DNA sample was found would make it "extremely rare" to get such a strong profile days later.
After stabbing 77-year-old Betty Yates to death, the person who murdered her stayed at her home to tidy up the "very neat" cottage a court has heard.
A walking stick used during the attack was placed back in its holder, a blood-stained book was placed on a desk and a medication packet, along with other blood-stained items, was put into a wicker basket.
– Forensic scientist, Padraig O'Shea at Bristol Crown Court
There was a similar degree of wear between the shoe and the print. There was a damage feature similar in shape and size to that on the boot. In my opinion, there is very strong support for the proposition the marks were made by the boots of Mr Farrow.
The man accused of murdering retired teacher Betty Yates in Worcestershire had previously declared he'd killed before, a court has heard.
The jury was told that Stephen Farrow had pleaded guilty to aggravated burglary at an address in Stourbridge in the West Midlands in 1994.
The victim, who was 77 at the time but has since died, opened her front door to a man who produced a knife with a "12-inch blade".
In her statement (parts of which are disputed by the defence counsel), Ms Crow said:
"He asked for money and jewellery. Then he said that if anyone came in, he would kill me and the dogs. He said, 'I have killed before'.
"He was holding the knife above my head in a stabbing stance."
Mr Farrow is also accused of killing Reverend John Suddards who was found murdered in Gloucestershire six weeks after the body of Betty Yates was found.
He admits the manslaughter of the Rev Suddards on the grounds of diminished responsibility but denies murder. He also denies killing Betty Yates.
Scientific experts are expected to give evidence next week.
The trial has begun of a man accused of murdering a retired schoolteacher in Bewdley in January.
Betty Yates was found stabbed to death in her home.
Stephen Farrow is accused of killing her, and the Reverend John Suddards who was found murdered in Gloucestershire six weeks later.
He denies both counts of murder, but admits to the manslaughter of Mr Suddards.