A 27-year-old man suddenly died while out walking his dog.
Ryan Johns was unaware that he suffered from hypertrophic cardiomyopathy, a heart condition that caused him to collapse during a stroll with his dog Dexter.
The dad was later found by a member of the public in Plympton, Devon, The Portsmouth Herald reported.
Speaking before Ryan's inquest on Monday, his sister Lauren John said her family were having to be tested for the heart condition.
It causes a portion of the heart to thicken, making it difficult for it to pump blood effectively.
"Ryan was fit and healthy, he never stopped and was always out and about," she said.
"He never showed any signs of having a cardiac condition.
"Ryan complained of a sore shoulder and arm several hours before he died, but he had been doing motocross during the day so it wasn’t too suspicious."
Shortly before he died Ryan went to his GPs with a chest infection and complaints of being short of breath.
It may be that some of his symptoms were mistaken for the flu.
Hypertrophic cardiomyopathy is inherited and cannot be prevented, with cardiac arrest occurring in around 1% of sufferers.
If the condition is detected early then treatment can be set up to prevent complications.
"The part I find most upsetting is I know how heartbroken Ryan would be leaving his son behind," Lauren said.
"The best thing we can hope for now is to raise awareness so this doesn’t happen to another family."
Ryan has been compared to "Tigger" as he was a "giant ball of energy and full of enthusiasm for life".
Lauren added: "He was a fantastic dad, going above and beyond to ensure his son Riley was always having fun. He was a kind and considerate son and brother.
"He had a true heart of gold, going out his way to help others.
"Riley was Ryan's life. They enjoyed motorsports together, bike rides and genuinely being out and about.
"Ryan thrived in an environment with his friends and often looked forward to family and friends holidays, weekends away, BBQs, fishing and beach trips."
The family have set up a fund for Riley, which he will be able to access when he is an adult.
Ryan, son of Nova and Darren, had one older brother, Jamie, and three younger sisters, Lauren, Kya and Neve and attended Ridgeway School.
"Education was not Ryan’s strongest attribute, certainly not the class angel, but winning over teachers' hearts with his cheeky personality," Lauren said.
Riley's mum, Abigail Slight, paid tribute to her "childhood sweetheart".
The pair met when they were 11 and became parents to Riley at 17.
She said: "He never shied away from his responsibility of being a parent.
"He was hard working and had a heart of gold which made him massively popular. He had friends from all different back grounds and lifestyle, he saw a person for who they were deep down and made his own judgements.
"There are not many people like that anymore. Ryan was fun, wild and loving."
She said that even when they split up, they always "maintained a great relationship".
"Having a healthy co-parenting relationship was important to us for Riley," she said.
