R. Haskel Lookstein relates a discussion between him and R. Joseph B. Soloveitchik (link):
A particularly poignant exchange between us occurred after I had attended a Rabbinic Alumni convention and heard a very prominent rabbinic scholar discuss the issue of Tay-Sachs testing. That scholar advised against any kind of testing for reasons which he explained. Furthermore, he said, once a woman was pregnant there is certainly no reason to test because even if the fetus were found to have Tay-Sachs disease there is nothing that can be done about it. One may not abort. I was concerned about that approach and, as luck had it, I had been invited to have dinner that evening at the home of a member of the Rav's family at which the Rav was going to be present. He was scarcely in the door when I described to him the view that had been expressed earlier that day and I asked him what his opinion was. He said very firmly: "You can abort a Tay-Sachs fetus through the sixth month." I said nothing but he must have noticed a quizzical look on my face as if to say - which, of course, I would not - what was the basis for the p'sak? I will never forget what he told me. He said: "Chatzkel, did you ever see a Tay-Sachs baby?" I replied that I had not. He said: "We had a Tay-Sachs baby in Boston. I tell you that you can abort a Tay-Sachs fetus through the sixth month." I saw at that moment a gadol in action, deciding a difficult question of Jewish law with absolute confidence and courage, based upon his scholarship and experience.