Andrew and Olivia Densley were faced with every parent’s worst nightmare when their little boy was diagnosed with a fatal genetic illness – Wiscott Aldrich Syndrome.

The doting parents of four had no idea they carried – let alone could have passed on – such an aggressive time bomb that effectively switched off Cooper’s immune system and left him in excruciating pain.

‘Gamble of Life’ airs tonight on Channel 9 after The Block. For more on 60 Minutes, head to the official website .

But Andrew and Olivia were determined to save their boy.

Cooper’s only cure was a bone marrow transplant and, by a pure miracle, his younger brother Jackson was an exact genetic match and could provide the lifesaving transplant.

Andrew and Olivia Densley were faced with every parent’s worst nightmare when their little boy Cooper was diagnosed with a fatal genetic illness – Wiscott Aldrich Syndrome. (60 Minutes) (60 Minutes)

Finally, after years of hell, their children were all happy and healthy.

Then Olivia fell pregnant again.

The parents had to make a terrible choice - terminate their unborn child or risk another baby inheriting their genetic curse.

Tonight on 60 Minutes, reporter Tom Steinfort is invited into the Densley family home as they share their unique story.

Theirs is a moral minefield that has many questioning – should Olivia have let herself fall pregnant again?

The doting parents of four had no idea they carried – let alone could have passed on – such an aggressive time bomb that effectively switched off Cooper’s immune system and left him in excruciating pain. (60 Minutes) (60 Minutes)

Given the traumatic ordeal of Cooper’s diagnosis, it came as a shock to many the Densleys rolled the dice by chancing to have another baby.

“I just felt like surely we wouldn't be that unlucky to get it again,” Olivia told 60 Minutes in a preview for this week’s story.

“I just had hope.”

On May 19, 2014, Fletcher came into the world.

Running out of time, Olivia and Andrew made a shocking move – they decided to have another baby. But this time, it wouldn’t be a surprise. (60 Minutes) (60 Minutes)

He was diagnosed with Wiscott Aldrich Syndrome just three weeks later.

“It was heartbreaking,” Olivia said.

“We know what he’s going to go through and I guess I did have a little bit of guilt of 'we did this to him'.

“It was a bit of a reality of, ‘What did we do?’”

This time, there was no miracle cure.

Arguably the most complicated family in Australia, their ever-growing family has been compared to the Hollywood blockbuster “My Sister’s Keeper”. (60 Minutes) (60 Minutes)

No match was found between Fletcher and his siblings.

Running out of time, Olivia and Andrew made a shocking move – they decided to have another baby.

But this time, it wouldn’t be a surprise.

They genetically engineered their sixth child to be a perfect match for their fifth.

Arguably the most complicated family in Australia, their ever-growing family has been compared to the Hollywood blockbuster “My Sister’s Keeper”.

Fletcher was diagnosed with Wiscott Aldrich Syndrome just three weeks after he was born. (60 Minutes) (60 Minutes)