They found out when their beloved daughter Mackenzie was diagnosed with the terminal condition at 10 weeks old.
She died about five months later.
"We had a beautiful life with her and we're thankful for every minute," Mrs Casella told AAP.
"But we had to watch our little girl die and we wanted to know why."
Mrs Casella and her husband were never told they could undergo genetic carrier screening before conceiving.
The couple has spent the past six years campaigning for more accessible and affordable testing so other babies won't unexpectedly be diagnosed with terminal conditions.
That goal became one step closer on Wednesday, with Australians now able to have bulk-billed genetic carrier screening for cystic fibrosis, spinal muscular atrophy and fragile X syndrome.
It previously cost would-be parents anywhere from $350 to $2200 to undergo testing, Victorian Clinical Genetic Services' Alison Archibald said.
"The majority of people who are having testing are from inner metropolitan suburbs that generally tend to be fairly affluent areas," Dr Archibald told AAP.
"When you put a Medicare item number in and you take away that barrier of cost of testing, what we hope we'll see is that people from all over Australia will be in a position to access that testing."
An organisation called Lumi Health is also making the tests as simple as a mouth swab that you can order online and have delivered to your door.
The program is the brain-child of myDNA Australia, Victorian Clinical Genetic Services and the Murdoch Children's Research Institute.
Mrs Casella, who is an ambassador for the program, said Lumi removed even more barriers for people who could't afford to go to a GP.
"I'm literally dancing out of my skin with the excitement," she said.
While not every prospective parent will want to have their genetics tested, Dr Archibald said giving them the choice was important.
"Having this information allows them to be informed, to know what those genetic chances are, and then to use that information to inform what reproductive decisions they make," she said.
There's hope bulk-billing will be expanded to testing for other genetic conditions.
"It's really the tip of the iceberg and there's a very large number of other significant genetic conditions that could be included," Dr Archibald said.
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