A UK forensic scientist says it is 7.2 million times more likely than not that Hayley Dodd contributed to DNA found on a strand of hair in the car her alleged murderer was driving the day she disappeared.

Andrew McDonald was giving evidence at the trial of Francis John Wark, who is accused of abducting and murdering the 17-year-old in WA's Wheatbelt region 18 years ago.

Ms Dodd was last seen alive in July, 1999, walking along a road near a property that Mr Wark owned at Badgingarra.

After she disappeared, the borrowed ute Mr Wark had been driving was forensically examined and debris on the floor was vacuumed up and put into a sealed container.

The contents was examined again 14 years later, when a single strand of hair was discovered by a scientist at PathWest.

In November 2015, WA Police took the root part of the hair to the United Kingdom for analysis by Mr McDonald, who testified on Wednesday in WA's Supreme Court about the complex scientific process he had used to test it.

He told the court levels of DNA were very low, but he concluded that Ms Dodd could have contributed to it

Mr McDonald said he also found traces of male DNA on the hair, which he said could be explained by the way it was stored for 14 years with other debris from the ute floor, or by a "drop-in" event that happened during the testing process.

He said after using specialist computer software to analyse the results, he came up with a "likelihood ratio" that it was 7.2 million times more likely than not that Ms Dodd and an unknown male, through "a drop-in event", had contributed to the DNA.

"Seven point two million would meet the requirement of extremely strong support that Hayley Dodd has contributed to the DNA evidence from the hair."

Mr McDonald said he also examined a DNA sample from Mr Wark, but his testing "gave no clear indication that he had contributed to the result".

Mr McDonald testified he was aware separate testing done on the shaft of the hair had also found DNA from Ms Dodd.

"To have two of the same I think is significant, it gives greater credence to my belief that this could be a mixed profile."

Mr Wark denies any involvement in Ms Dodd's disappearance and is challenging the DNA evidence, claiming it is flawed and should not be accepted.

Under cross examination from Mr Wark's lawyer, Darryl Ryan, Mr McDonald agreed that once DNA was mixed together it became "difficult if not impossible" to determine where it came from

"It's a bit like putting M&Ms into a jar and figuring out which M&Ms came from different people?" Mr Ryan asked. Mr McDonald replied: "Yes … the more people who contribute, the harder it becomes to know."

Prosecutors say hair and earring link Wark to crime

The earring found in the car seat cover of the vehicle Francis John Wark was driving. ( Supplied )

The hair is one of two physical pieces of evidence that prosecutors say link Mr Wark to the crime.

The other is an earring which was found in a car seat cover that had been seized from the ute in 1999, but which was not discovered until 14 years after Ms Dodd's disappearance.

Prosecutors say the earring is similar to sketches done by a friend of Ms Dodd's of the jewellery the 17-year-old was wearing the day she disappeared.