But how would we know that someone has that disease? If somehow you could know that the two parents are both carriers of the Tay-Sashs disease then maybe you would test if the DNA of the fetus is miss in the Hex A enzyme but otherwise there would be not really be any other way. DNA test are also decently expensive so unless technology will develop to the point where we can easily aqquire our DNA results at a low or no cost, I don't think that doing genetical engineering will work.