But there’s something about the precision of a DNA test that can make people believe that chemistry is destiny—that it holds dark, implacable secrets. This is why genetic information is sometimes described as "toxic knowledge": Giving people direct access to their genetic information, in the words of Stanford bioethicist Hank Greely, is out and out "reckless."

We are all vulnerable to particular disorders, and may have predispositions to cancer or heart disease or dementia, even if we currently feel perfectly healthy. That's why genetic information has been called "toxic knowledge", because it may have corrosive effects on our sense of self, our relationship with our family, or our economic and social wellbeing.

Amid talk of mouse models, intermediate alleles, and subcellular expressions of the protein huntingtin, the presence of those who have tested positive for the Huntington's gene creates a special atmosphere, a powerful aura, the haunted future we are trying to disrupt. How are people living with this "toxic knowledge," as it has been called?