Tay-Sachs carriers

Leonard, 25, and Rachel, 23, discover that they are Tay-Sachs carriers after Rachel has become pregnant, and the doctor informs them that they have a 1 in 4 chance of bearing a Tay-Sachs child. Tay-Sachs is a fatal disease that is both degenerative and particularly horrible. They decide to wait until the fourth month of Rachel's preganancy to have an amniocentesis performed. The results, which they receive in Rachel's fifth month, show that she will indeed give birth to a child with Tay-Sachs. What should they do? Would abortion be morally acceptable in this scenario? Does the concept of an "informed conscience help in this situation? What would you do?