Most genetic diseases only occur when two parents happen to have it, but they won't necessarily be aware of it; would it be unethical for people who are unaware of their genetic defects to have children?

Second, according to a quick search, 10% of cases of Huntington's Disease are due to new mutations; I suspect (but I'm a HN commenter, no geneticist) this is the case for many other genetic conditions.

So the other ethics question to ask: should people be able to get DNA tests for genetic conditions (voluntary)? I'd say yes. Should people be mandated to get DNA tests and be forbidden to procreate if there's something in there? No, that's eugenics. Should people who know they have a genetic condition and there's a chance their child has it too have children? That'd be their choice. I don't think it's fair for people to intentionally place a burden on health care systems like that, but thing is, there's very, very few people that have children with that as the intent.