Aware she could be a "carrier" of the gene mutation and pass it onto her children or grandchildren, she asked her GP to organise chromosomal testing. She was told the result of the testing was negative.

Believing she was not a carrier, Mrs Eastbury and her husband Phil had a son, Hayden, in 2008. A second son, Jacob, was born in 2011.

After Hayden failed to meet his development milestones, blood and DNA tests were carried out, which showed Hayden has the full Fragile X gene mutation. Further testing showed Jacob also has the full mutation and Mrs Eastbury is a carrier.

The couple are suing prominent fertility specialists Genea, formerly known as Sydney IVF, in the NSW Supreme Court for economic and non-economic loss as well as for mental harm.

After the initial devastation at the diagnosis and the realisation the pre-conception testing had mislead them, the couple say they are determined to do whatever they can to help the boys cope with the intellectual disabilities, language, emotional and behavioural problems caused by the incurable condition.

Mr Eastbury says Hayden and Jacob's senses are amplified, so whereby most people can filter out background noises and visual stimulation, they boys become overwhelmed, leading to anxiety and meltdowns.

"Things such as hand dryers in public toilets, vacuum cleaners and the sound of water going down the bath drain upsets them," Mr Eastbury said.

"Large crowds and unfamiliar places can cause either, or both of them to meltdown. We find it hard to go and do anything out of the boys' normal routine, such as holidays, day trips away or visiting friends."

A regime of early intervention techniques including speech and occupational therapy and chiropractic treatment help the boys regulate the senses. Hayden receives extra support in the classroom at school and Jacob attends an early-intervention pre-school program.

An August 2014 study by the National Centre for Social and Economic Modelling found, on average, a child with Fragile X syndrome will cost about $200,000 more to raise to age 25 than a child without Fragile X syndrome.

The genetic testing took place in 1999, meaning the couple could have been outside the three-year limitation period to bring a law suit. Last month Justice Peter Hall granted the Eastburys an extension, despite Genea's opposition on the grounds it would suffer prejudice.

The court heard in September 1999 Mrs Eastbury's GP wrote a referral to Macquarie Pathology requesting "genetic testing for carrier status of X-factor". The referral noted her "uncle has mental retardation."

A blood sample was taken and referred on to Genea, then known as Sydney Genetics. However, at that time, Genea did not perform testing to establish the carrier status of Fragile X. The only testing it performed was chromosomal analysis to determine whether a person was currently affected by Fragile X.

Justice Hall said if Genea didn't have the capacity to undertake carrier testing, it should have told Macquarie Pathology. Instead, it undertook a limited test analysis and reported the results as "Fragile X negative".

"Genea's report was effectively conveying or suggesting, or at least impliedly representing, in answer to [the GP's] handwritten request, that Mrs Eastbury had no carrier status", Justice Hall said.

Macquarie Pathology was deregistered in 2007 and all its documents have been destroyed.

The Eastburys say raising two boys with a disability is a steep learning curve and at times can be overwhelming, but "we are truly blessed to have them in our lives."

"Fragile X syndrome is just a small part of who they are – it doesn't define them," Mr Eastbury said.

"While having two boys with FXS is very challenging for us as parents, it is also a blessing because the boys have each other."

Fragile X syndrome occurs in approximately 1 in 3600 males and 1 in 6000 females.

The Fragile X Association of Australia says every week, one child is born who is fully affected and 20 children are born who are carriers. It is estimated that 5 per cent of people with a diagnosis of an Autism Spectrum disorder also have Fragile X.

Many people are unknowingly carriers of the condition. In Australia, one in 170 women and one in 800 men are carriers of the Fragile X gene, meaning they have the potential to conceive a child with Fragile X syndrome. It is not currently included in state newborn screening programs.

Children of either sex born to female carriers have a 50 per cent chance of inheriting the affected gene. If they inherit the affected gene they will either be a carrier or have full mutation Fragile X.

Boys and men are usually more severely affected than girls and women. Typical features include anxiety and shyness, autistic-type behaviours, repetitive speech, aggression and difficulty with communication, speech, fine and gross motor skills and coordination.

A spokeswoman for Genea said it was not appropriate to comment while the case was before the court. The case returns to court next week.