The problem I see is that, because such a low percentage of Americans purchase health-related insurance policies, even relatively small increases in propensity of those with diseases like Huntington's to buy-in would have huge impacts on the expected cost of an insurance company's policyholder pool.

According to this article, an individual can know with near certainty that he will become symptomatic with Huntington's disease in the future if certain genetic markers are present:

http://en.wikipedia.org/wiki/Huntingtons_disease

[I am clueless about most genetic diseases, btw]