One day in July 2004 ex-soldier Andrew Wragg visited his sick son and put a pillow over his head, suffocating him.
Mrs Wragg said Jacob was "happy and giggly" right up to his death
He phoned the police telling them it was a mercy killing to end Jacob's suffering due to Hunter Syndrome.
The murder trial which followed the killing saw the family split as relatives of both 38-year-old Wragg and his wife Mary gave evidence in court.
Their testimonies helped lead the jury to accept Wragg's claims he smothered his son to end the boy's pain.
Ten-year-old Jacob's incurable degenerative illness meant he was expected to die by the time he was in his 20s. He was also deaf and nearly dumb.
Branded a murderer
Jacob's mother told the jury her husband had spoken on a number of occasions about smothering their son.
She said that on a visit to see Jacob at a Manchester hospital, Wragg, from Worthing in West Sussex, spoke to the mother of another terminally ill child about their similar situations.
Mrs Wragg, 42, said she clearly remembered her husband telling the woman that if he thought Jacob was suffering unnecessarily he would "put a pillow over his face."
In Wragg's first murder trial, in which jurors failed to reach a verdict, leading to a retrial, Mrs Wragg branded her husband a murderer.
Accused by the defence of inventing stories to slur her husband's character she replied: "I do not have to assassinate my husband's character... he murdered my child."
In the retrial, Mrs Wragg said she "could not believe it" when her husband told her he had killed Jacob.
Mary Wragg and her mother gave different views on Jacob's condition
The court heard Wragg had called her to say he was going to "take Jacob away and kill him" and that he was "doing it for Jacob".
But Mrs Wragg said she did not believe he would really hurt their son and, although she was hesitant, felt it safe to leave them alone together in the house later the same day.
While she was out Wragg called to say he had ended Jacob's life.
She told the court: "He said he had killed Jacob. He said he had called the police.
"I could not believe what he was saying."
Despite Jacob's illness, the court heard he was "not at death's door" and Mrs Wragg maintained he was happy on the day he had died, when she had taken him to a funfair in Worthing, and treated him to a hot dog and chips.
She said: "He was happy and giggly and clapping his hands. He was jolly."
Andrew Wragg was described in court as a "wonderful father"
But Mrs Wragg's mother Gwendoline Richards told the court a different story.
She said her grandson's death had been "a release from being ill" and that although she had not wanted him to die, she had not wanted him to suffer.
The court heard that on hearing of her grandson's death, she told police: "To hear this was the best news I could have. He was not Jacob any more, he was just a shell."
If her words backed up Wragg's claims that killing Jacob was an act of mercy rather than murder, he was also supported by his brother and sister who told the court he was a "wonderful" father who had an "amazing" relationship with Jacob.
And the jury chose to agree with Wragg, the man who admitted the manslaughter of his son, by clearing him of the boy's murder.
Wragg walked free from court after the judge gave him a two-year suspended sentence for manslaughter.