William MacDowell, of Penrith, Cumbria, is accused of murdering his son Andrew MacRae and the child’s 36-year-old mother Renee MacRae in November 1976.
Gordon MacRae took to the witness stand at MacDowell’s trial at the High Court in Inverness on Wednesday.
Advocate depute Alex Prentice KC put it to Mr MacRae at the end of his evidence session: “Did you have any part whatsoever to play in the death of Renee MacRae or Andrew MacRae?”
The 85-year-old told the jury: “Absolutely none.”
MacDowell, 80, denies the charges against him and has lodged a special defence of incrimination and alibi, which states he was at home that evening and that Mr MacRae, along with others, committed the alleged crimes.
Murray Macara KC, cross-examining Mr MacRae, put it to him that he was “in some way complicit” in the disappearance of the mother and son on the night of November 12, 1976, which he denied.
Mr Macara told the court that Mr MacRae used to visit a nearby quarry with a then girlfriend and said to her: “If you fall out with me this is where you would end up.”
Mr MacRae described that as “absolute nonsense”, and repeated the denial when the prosecution KC put it to him that he said if there was a landslide there would be “hundreds of tonnes, thousands of tonnes of earth on top of the body”.
The MacRaes married on May 17, 1963, but they split in 1975. In July 1976, she moved into Cradlehall Park, near Inverness, in a home provided by her estranged husband.
The former director of Hugh Macrae Builders Limited said they did not have a brilliant marriage, but when they split they had an “amicable” relationship.
The court was told he knew she was involved with somebody else, but despite repeatedly asking if it was someone he employed at his family firm, she would not reveal who it was.
Mrs MacRae, of Inverness, has not been seen since the night of November 12, 1976, and her disappearance sparked a huge police investigation.
MacDowell is charged with assaulting Mrs MacRae and their three-year-old son at the Dalmagarry layby on the A9south of Inverness, or elsewhere, and murdering them.
He is also charged with disposing of their bodies and belongings by means unknown.
Mr MacRae told the jury on Wednesday that he asked MacDowell, who was company secretary, into the boardroom in the days following the discovery of a burnt-out BMW in the case, and after he found out it was MacDowell who was involved with her.
Mr MacRae said he told him: “If you can help the police in any way, please do.
“He didn’t say a word the whole time in the boardroom. Not a word.”
He then fired MacDowell because of the relationship and he has never seen him again.
Mr Macara asked Mr MacRae: “When police became involved did you at any stage feel police suspected you might be involved in Renee’s disappearance.”
Mr MacRae said he did not, and then told the court he remembered that police had asked to search his house.
The trial continues.