''If I didn't know, I would spend the rest of my life second-guessing decisions,'' she says.

The results confirmed that within the three billion letters of Johnson's genome, a single mistake means she has a 98 per cent chance of developing the neurological condition that killed her mother at 47.

''It scares the life out of me to think I'll not be able to communicate with people,'' says Johnson, now 33.

When the human genome was first sequenced 13 years ago, it was hailed as a medical revolution, the catalyst for this new era of personalised medicine.

For most of the past decade, however, genetic testing remained an expensive tool for research laboratories only.

But in recent years, the technology to read a person's genome has advanced at a staggering pace. Now a vial of spit or blood and an internet connection is all anybody needs to peer inside their body's blueprint.

But while labs can sequence a person's DNA in a couple of days for less than $1000 - compared with the original human genome project, which took a decade and cost more than $2 billion - interpreting a person's 23,000 or so genes into something meaningful for their future health has proved much more complex.

In a small fraction of diseases, such as cystic fibrosis, Huntington's disease and early onset familial Alzheimer's, genetic mutations are the direct cause. If you have the mutation (or mutations), there is no escaping the disorder.

In families with a history of such disorders, a genetic test can provide certainty to individuals such as Johnson, who show no symptoms of the disease, and can help them make personal decisions about their future.

For most diseases, however, the genetics are not so simple. In conditions such as cardiovascular disease, diabetes, stroke and most cancers, many genes are implicated. They are neither necessary nor sufficient to cause the disease by themselves (lifestyle factors are also important), but each contribute a small proportion of risk.

Until medical research works out with certainty the impact of these inherited defects, how they interact with the environment, and if anything can be done about them, the benefits of genetic testing for most people are unclear.

For about a year and a half after Chontell Johnson discovered she carried an Alzheimer's mutation, the beautician trainer, a life-long planner, stopped preparing for her future.

''I ate great food, drank too much and I didn't save any money,'' she says.

Since then, Johnson has learnt to take each day as it comes.

''I would never have realised how strong I was until I [got tested] and made it through,'' she says.

Professor Peter Schofield, a geneticist and head of Neuroscience Research Australia, says research is challenging the perception among some clinicians that people cannot cope with the knowledge they carry a deadly gene mutation.

Studies by Schofield and others have found many people are open to genetic testing, and that many who went ahead with the sequencing and discovered they carried a mutation for a dominant genetic disorder did not suffer significant psychological distress in the short term.

''We found people were amazingly resilient to testing,'' Schofield says. It removed the uncertainty, and allowed people to make choices about their future care, he says.

But certainty made some decisions, such as whether to have children, gut-wrenchingly hard, Johnson says.

''I would like nothing more than to have three beautiful children and be a great wife,'' she says.

''But I watched my mum die, and it was excruciating. To know that is my future and to consider my children would have to watch me in that situation is huge.''

Genes, and some mutations, are what make families into a family. So the decision to get tested has implications beyond the individual having their genome sequenced.

A clinical geneticist, Professor Martin Delatycki, says only 15 per cent of people with a family history of dominant genetic diseases such as Huntington's, where offspring have a 50 per cent chance of inheriting the disorder from an affected parent but there is no cure or treatment, choose to learn if they carry the affected genes.

Part of Delatycki's job is preparing people for their test results, and then counselling them on what they mean.

''Some people, you tell them they've got a 50 per cent chance of getting cancer and they say, 'Fantastic, that means there's a 50 per cent change I won't get it,''' says Delatycki, the director of clinical genetics at Melbourne's Austin Health.

''Other people, I tell them they've got a very small risk of cancer and they find it very difficult to deal with,'' he says.

''Perception of risk is very personal.''

Zenon Slotwinski manages risk for a living. As a finance treasury manager, he knows the advantage of finding a faulty investment early. So when doctors discovered his mother's pancreatic cancer was partly caused by a mutated BRCA2 gene, he and his sisters got tested.

''Unfortunately, we have a bad track record, and we all have it,'' he says.

The mutation means Slotwinski, 54, has a 5 per cent to 12 per cent greater risk of developing pancreatic or prostate cancer than the general population, while his sisters have a 15 to 30 per cent increased chance of contracting breast, ovary or pancreatic cancer compared with other women.

Based on their risk profile, Slotwinski and his siblings take part in regular screening programs. His sisters took the more dramatic step of having pre-emptive surgery - one to remove her ovaries, while the other had a complete hysterectomy.

Slotwinski says he has coped well with the knowledge of his increased risk of cancer, because with knowledge comes power. ''I like the idea of catching something early,'' he says.

Slotwinski and his sisters represent the group with the most to gain from genetic testing - people who have a known mutation linked to specific cancers but are yet to display any symptoms and can therefore be monitored.

Cancer geneticist Professor Graeme Suthers says clinicians encourage patients with a genetic mutation that is known to increase cancer risk to speak to their family about testing.

''It means we can target screening to those people who really need it - early cancers will be detected and lives will be saved,'' says Suthers, a spokesman for the Royal College of Pathologists of Australasia.

But despite the health benefits, only about 20 per cent of at-risk relatives come forward for testing. Suthers suspects it is a combination of fear and busy lifestyles that keeps people away.

In 2001, the Adelaide familial cancer clinic he directs trialled a different approach to boost the number of at-risk family members coming for testing.

The clinic sent letters to family members who might be at risk, offering them the opportunity to be tested.

''We wanted the letter to be sufficiently clear, so people didn't throw it away, and sufficiently confronting, so people would take notice, but not too confronting so people lay panic-stricken in their beds,'' he says.

The approach worked, and the number of at-risk family members who fronted up for testing doubled to 40 per cent.

''But it leaves begging [the question] why 60 per cent of this population in the 21st century will not come forward to obtain potentially life-saving information about their health,'' Suthers says.

The benefits of genetic testing for people such as Zenon Slotwinski (regular cancer screening), and Chontell Johnson (certainty) are well-documented, but for most disorders the genetics are complex and the benefits of testing less straightforward.

The human genome is a complex language, and while scientists have deciphered its basic code, they struggle to interpret its meaning.

''We now recognise some nouns, verbs and objects, but there is an awesome amount we don't yet understand,'' Suthers says.

While geneticists discover new gene mutations daily, it can take years to determine whether they cause disease, contribute to disease-risk or are harmless variations, Martin Delatycki says.

''I spend a lot of my time talking to patients about uncertainty,'' he says.

For these reasons, many researchers, including Suthers and Delatycki, are concerned by the rapid and unregulated rise of direct-to-consumer genetic tests.

The complex relationship between genes and disease can bewilder unqualified people who can easily misinterpret their test results, Delatycki says.

Genetic testing has consequences for life insurance, as people must declare if they are at risk of certain diseases.

Ethicist Jacqueline Savard says a lot of genetic information is meaningless to people's everyday lives, and society risks creating a population of ''worried well''.

''Sometimes you can know too much,'' says Savard, who is completing a PhD at the University of Sydney on people's perceptions and expectations of genetic testing.

Some people will place too much emphasis on the idea they might get a disease rather than living their life, she says.

But as the US geneticist Francis Collins said in 2010 on the 10th anniversary of the human genome project he helped to lead: ''We invariably overestimate the short-term impacts of new technologies and underestimate their longer-term effects.''

For people such as Chontell Johnson, these longer-term possibilities are the difference between life and death.

''I'm not living in the hope something will happen, but I am hopeful every minute of my life,'' she says.