Why you shouldn’t know too much about your own genes

This summer, a panel of genetics experts did something surprising: they put out a list of genetic tests people should not get.

In the age of precision medicine, the genome is our oyster. There are cancer wonder drugs that pinpoint the errant genes that drive tumors. There are longstanding medical mysteries finally being unraveled by DNA sequencing. There is tremendous excitement over the coming age of treatments tailored to you. And there is also this: a very long list of genes for which the best medical understanding of what they mean for our health is essentially a shrug.

The poster child for the uncertainty underlying much of the information in this brave new world is a gene called MTHFR. It produces an important enzyme, but many medical geneticists simply sigh when they hear the gene's clumsy acronym name.

This gene has made its way on to the do-not-test list more than once, because in almost no cases do the tests have any medical utility. Meanwhile, alternative medicine practitioners and Web sites have stepped up their claims that many people --...