What is wrong with eliminating genetically-based disability?

As our understanding of the genetic structure of our species has advanced, and our technological ability to manipulate that structure has become increasingly sophisticated, so we are increasingly able to screen foetuses for genetically-based conditions. That much is uncontroversial. What is controversial, however, is what, if anything, it is morally right to do with this knowledge and this ability. On the one hand, to prevent, if we can, people being born who will suffer grievously on account of a condition brought about by their genetic make-up would seem at least morally permissible, if not morally required. On the other hand, if we do so, then what does that say about our attitudes to and treatment of currently living people who suffer from the condition in question? I conclude that where the conditions concerned really do constitute disability; and where such disability cannot be avoided by changing social norms, or where it should not be avoided by changing them, so that the condition is no longer perceived as a disability but as normal: under these circumstances, eliminating disability devalues no one, whether that real disability is social, educational, physical or mental