By Robert Bazell,
Chief Science and Health Correspondent
NBC News

A readout of all of your DNA? You’ll get it soon. Many wealthy people have theirs already, but within years because of ever-accelerating technology it will be cheap enough for all, including every baby at birth.

So what will this “genetic blueprint” tell us of our future health?  Not much, according to an important study out Monday from a group of scientists at the Johns Hopkins University School of Medicine. They analyzed thousands of identical twins. Twins share the same genes from birth, so their experiences illustrate the insights to be gained from any individual knowing his or her genetic future.

The scientists looked at 24 common diseases and found that occasionally the complete genetic blueprint will alert people that they run a risk far higher than others for a certain disease. But most will get very little useful knowledge. The scientists presented their work at the annual meeting of the American Association for Cancer Research and published it simultaneously in the journal Science Translational Medicine

“Genomic tests will not be substitutes for current disease prevention strategies,” said Dr. Bert Vogelstein, one of the authors and a pioneer in the search for genes that increase cancer risk. 

So a blood test of the future will not free you from the need to eat healthy, exercise, keep extra weight off, not smoke and get useful cancer detection tests such as pap smears and colonoscopies. Nor will it relieve the ongoing possibility of nasty surprises about diseases you may have never feared.

But isn’t our genetic blueprint our destiny? Many scientists thoroughly believed that not long ago. As they have learned more about genes, however, that prediction appears ever more simplistic. 

Most diseases arise from a complex mixture of the genes we inherit from our parents at birth— not only what is measured in the whole gene test, but also our lifestyle and environment, and random events such as gene mutations occurring in individual cells in our body later in life.

One of the best examples of the dilemma of genetic testing comes from a test that has been on the market since 1996 for the cancer mutations called BRCA1 and BRCA2

If a woman is born with one of those mutations she has a 35 percent to 84 percent chance of developing breast cancer by age 70 and a 10 percent to 50 percent risk of getting ovarian cancer. Those odds are so high that women who test positive often elect to have their ovaries and/or breasts removed before cancer strikes. In those situations the test saves lives. But the problem is that fewer than one in 1,000 women carry those gene mutations.

And if women test negative for the gene mutations, the negative test does not at all mean she is in the clear. There are so many factors that increase risk for breast and ovarian cancer that a woman who tests negative for BRCA1 and BRCA2 only reverts to having the same lifetime risk as all other women – a 12 percent risk for breast cancer and 1.5 percent for ovarian cancer.

That set of complexities mirrors the situation for the thousands of other genes we carry that have some association with a disease.  If everyone has a complete gene profile, a small number can learn they have a great risk for something.  But for most, the information is minimally significant.

Not long ago many investors saw whole gene sequencing as a future gold mine. But these latest results show that the tests are unlikely to make many people wealthy -- or healthy.

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Robert Bazell is NBC's chief science and medical correspondent. Follow him on Facebook and Twitter.