March 18, 2013, 3 a.m.

There was no doubt in Chontell Johnson's mind. When a genetic test revealed her mother had early onset Alzheimer's, the then-27-year-old decided she would get tested, too.

''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.

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Living with a time bomb

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