Genomics is an exciting and important science. It is radically altering our understanding of biology. But the genetic component of the most serious diseases isn’t nearly as large as you might think.
Take colon cancer as an example. It kills some 50,000 Americans each year, making it the second most common cause of cancer deaths. Without question, there is a genetic component of risk for colon cancer, and this risk is well-established to be statistically significant.
But “statistically significant” does not mean large. It doesn’t even mean moderate. The ten most significant genetic variations that affect colon cancer account for 1.3% of lifetime risk . That’s not 1.3% on average, that’s 1.3% collectively, all summed together. The clinical utility of having your genome analyzed for colon cancer risk is zero. And the risk of having a procedure done to “repair” any mutation would surely be greater than the benefit.
Colon cancer is no outlier. Twin studies give us a pretty good idea of genetic contributions to major diseases. It is remarkably small. This figure from a large study of twins in Western Europe shows just how small:
The red portion of the bars indicates the number of deaths attributed to genetics plus shared environmental exposures (twins share not only genes, but much of their environment, especially in childhood). The red portion is thus an upper limit on the contribution of genetics.
There absolutely are diseases that could benefit from genomic repair. The BRCA mutations raise a woman’s lifetime risk of breast cancer from 12% to 70% . However, BRCA mutations are rare, occurring in only 0.1–0.5% of the general population . They account for 3–7% of overall breast cancer risk. That’s not nothing. But eliminating all BRCA-associated cancers would put only a modest dent in overall breast cancer incidence and mortality.
The bottom line is that even the most advanced and perfected gene therapies will never have much of an impact on human health. Their potential is trivial compared to what we already know to be more helpful: quitting smoking, reducing obesity, getting exercise, improving air and water quality.
“Free” healthcare (which doesn’t exist) is in no danger from gene therapy.
 New insights into the aetiology of colorectal cancer from genome-wide association studies.
 Genetic Factors Are Not the Major Causes of Chronic Diseases
 Study Affirms Cancer Risk for Women with BRCA Mutations
 RECENT ADVANCES IN BREAST CANCER GENETICS