A FORENSIC scientist has told the World's End murder trial there is a less than one-in-a- billion chance DNA on the coat of murdered teenager Helen Scott could have come from anyone other than the accused, Angus Sinclair.

Martin Fairley, 52, a scientist with the Scottish Police Authority, told the jury at the High Court in Livingston it was a conservative estimate, and the chance could be as low as one-in-three-billion.

The tiny semen stains on the coat had between them yielded enough forensic material to produce a DNA profile with 17 out of 20 regions identified. All 17 matched the profile of Sinclair, 69, so although the profile was not complete it was compelling.

Mr Fairley also told the jury his examination of intimate swabs from Miss Scott's friend, Christine Eadie, who Sinclair is also accused of murdering, had yielded a major contributor, and some had also revealed a minor contributor.

In all cases, Sinclair's late brother-in-law Gordon Hamilton was revealed as the major source of semen found on Miss Eadie's body. But the swabs had also produced partial profiles with DNA types that matched Sinclair. In one case, Mr Fairley estimated there was a one-in-560 chance the source was someone unrelated to Sinclair, and in another a one-in-40 chance.

Cross-examined by Ian Duguid QC, defending, he accepted DNA types identified from swabs taken from Miss Eadie's body, which he had described as unique to Sinclair during his evidence in chief, had been shared with Miss Scott.

Mr Duguid suggested someone having sex with Miss Scott and later with Miss Eadie might have cross-transferred some of Miss Scott's DNA to Miss Eadie's body, reducing the significance of the partial profile.

Mr Fairley rejected the "hypothesis" as extremely unlikely, reminding the jury the DNA had come from seminal fluid, which could only be male.

Re-examined by the Lord ­Advocate, Frank Mulholland QC, prosecuting, Mr Fairley was asked to comment on another suggestion made by Mr Duguid that the swabs could have been contaminated in the laboratory. Mr Duguid had pointed in particular to a DNA type shared by Sinclair and Professor Ken Mason, one of two pathologists involved in post-mortem examinations carried out on the bodies on October 17, 1977.

Mr Fairley said: "I find it, if not impossible, almost impossible to countenance."

Earlier Susan Ure, 42, another forensic scientist, completed her evidence and also faced challenges from Mr Duguid about possible cross-contamination between the parties involved in the case, and contamination in various laboratories.

Under re-examination by the Lord Advocate, she was asked about the likelihood that partial profiles that matched with Sinclair might have been distorted by DNA types he shared with the victims in the case. She replied that the lab work had concentrated on sperm found on the swabs. She added: "We would not expect female DNA to be present."

Miss Eadie and Miss Scott, both 17, were last seen leaving the World's End pub in Edinburgh's High Street on the night of ­October 15, 1977. Miss Eadie's body was found next day on the beach at Gosford Sands, East Lothian. Miss Scott's was found later the same day four miles away in a wheat field at Coates Farm, near Haddington.

Sinclair denies raping, battering and murdering Miss Eadie and Miss Scott while acting along with the late Gordon Hamilton. He has lodged three special defences: incriminating Hamilton for the murders, claiming consent to any sexual activity and an alibi claiming he was fishing at the time of the murders.

The trial continues.