My thinking is that more genetic diversity tends to be better for a species. Incidents like the Irish potato famine show that monoculture is a Bad Thing. And greater genetic diversity means more evolutionary paths are pursued, possibly leading to more advantageous traits emerging in the population.

Genetic diversity can result in what is sometimes called "hybrid vigor." An example that is sometime cited is that mixed breed dogs (good old mutts) often have less genetic problems than pure breeds.

What I'm wondering about is whether some of the genes that survive to be passed on are a good thing or not. For example, suppose someone has a genetic predisposition towards a severe childhood cancer. In the past they likely wouldn't have survived to pass on those genes. Medical advances might allow them to survive their cancer today and to become parents. Is passing on those genes necessarily a good thing?

Just to be clear, I'm not suggesting in any way that we shouldn't treat childhood cancer or that those who survive childhood cancer shouldn't have children of their own. I'm just wondering if passing on certain genetic problems is necessarily a good thing for humanity in the long term.