Dworkin hits the nail on the head in Sovereign Virtue:
“My hypothesis is that genetic science has suddenly made us aware of the possibility of a similar though far greater pending moral dislocation. We dread the prospect of people designing other people because that possibility itself shifts… the chance/choice boundary that structures our values as a whole, and such a shift threatens, not to offend any of our present values… but, on the contrary, to make a great part of these suddenly obsolete.
….The terror many of us feel at the thought of genetic engineering is not a fear of what is wrong; it is rather a fear of losing our grip on what is wrong. We are not entitled–it would be a serious confusion–to think that even the most dramatic shifts in the chance/choice boundary [the distinction between what we are and are not responsible for] somehow challenge morality itself: that there will one day be no more wrong or right. But we are entitled to worry that our settled convictions will, in large numbers, be undermined, that we will be in a kind of moral free-fall, that we will have to think again against a new background with uncertain results.”