Teeth marks found on Robin Bain's right hand linked him to a fight with his younger son the morning five family members were killed, a High Court jury has been told.
The Crown says David Bain, 37, killed his parents Robin and Margaret and siblings Arawa, Laniet and Stephen at the family's house in Dunedin on 20 June 1994.
The defence says Robin Bain killed the family present in the house before shooting himself.
On Monday, a dental surgeon testifying for the defence in the retrial said the teeth marks link Robin Bain to a fight with his son Stephen Bain on the morning he was killed in 1994.
Donald Adams, a maxillo-facial surgeon and forensic dentist, told the court that four marks on Robin Bain's knuckles matched the teeth imprint of a teenage boy.
Mr Adams said the marks could have been caused if Robin Bain had struck Stephen Bain with an upper cut to the teeth.
But in cross-examination he conceded the marks could have been caused by something other than teeth.
Earlier in the trial, the court was told there had been a violent struggle in the 14-year-old boy's bedroom on the morning of the killings as Stephen Bain fought for his life.
The defence says Robin Bain killed Stephen Bain and the other family members before turning the gun on himself.
However, the jury has also been told of scratches and abrasions on David Bain's chest, head and knees, which the accused could not explain.
Father may have shot himself - pathologist
A fifth pathologist to give evidence at the retrial has concluded that Robin Bain's death could have been self-inflicted.
Professor Stephen Cordner from Melbourne, giving evidence via video link on Monday, said he examined the photographic evidence of the wound to Robin Bain's head and regarded it as a contact wound or close contact wound, meaning the rifle was held against his head.
One of the three Crown pathologists has previously told the jury he considered the shot to have been fired from a distance of up to 42 centimetres.
