scifi: that would be far far away in the future, when too many people with direct-gene defect mutations will have had children. But with the complexity of genetics, all that may be pointless: beyond our understanding combination of genes will trigger a disease once some conditions/imbalance are/is met in some environments with some specific history. Humanity may end up as a set of clones of "known" stable genetics over the long run and environments with a "normal" history.