The High Court in Livingston was told yesterday that the chances of it coming from another man unrelated to the accused, Angus Sinclair, was around one in a billion.
A forensic scientist reached the conclusion – described as a “conservative estimate” – after analysing staining on Helen Scott’s coat, jurors heard.
Sinclair, 69, denies raping and murdering Helen and her friend Christine Eadie, both 17.
The pair were last seen at Edinburgh’s World’s End pub on 15 October 1977. Christine’s body was found the following afternoon at Gosford Bay, East Lothian, while Helen’s body was discovered in a wheat field near Haddington.
The ninth day of the trial heard evidence from forensic scientist Martin Fairley.
He told how he analysed four small areas of white staining taken from the inside lining of Helen’s coat for DNA. A profile consisting of 17 types or regions of DNA was obtained and found to match 17 of the DNA types in Sinclair’s profile, the court heard.
Mr Fairley concluded: “The probability of finding these 17 DNA types matching types in Angus Sinclair’s DNA profile, if the semen staining from the lining of the coat worn by Helen Scott originated from another male unrelated to Angus Sinclair, is estimated to be in the order of one in a billion.”
The court was also told about swabs taken from Christine’s body which produced partial DNA profiles.
In one, the chances of it coming from another man unrelated to Sinclair was estimated as being around one in 560. In the second, it was said to be around one in 40, the court was told.
Sinclair has submitted three special defences of incrimination – blaming his brother-in-law Gordon Hamilton, now dead; alibi – saying he was fishing on the banks of the Firth of Forth near Cockenzie power station at the time; and consent to sexual intercourse.
However, the court also heard suggestions that the process of separating DNA from other material contained within a sample may not be “100 per cent foolproof”.
Sinclair denies repeatedly punching and kicking Christine or otherwise inflicting blunt force injuries, and biting her.
He further denies forcing Helen to walk barefoot into a field, repeatedly punching and kicking her on the head and body and stamping on her head.
It is alleged he bound their wrists and tied a ligature around their necks, which he denies.
The trial continues.