Resilient: Chontell Johnson is at risk of Alzheimer's. Photo: Danielle Smith.
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.
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.
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.
Labs can sequence a person's DNA in a couple of days for less than $1000.
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.
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