A progressive neurodegenerative disorder. Untreatable, uncurable, but detectable by specific tests.
Children are born normal. By six months of age they have begun to show signs of progressive motor and mental degeneration. They die by age 2-4. Always. Untreatably.
Prevalence: 1 in 30 Ashkenazi Jews is a carrier of the disease. If two carriers get married, 1/4 of their children will have Tay-Sachs.
We have the ability to tell if someone is a carrier of Tay-Sachs...and to then test the child of two Tay-Sachs carriers to see if the child is afflicted. At that point, about 95% of couples elect to terminate the pregnancy.
I think the fact that 99% of couples who'd previously had a child with Tay-Sachs decided to abort the second child with Tay-Sachs is a telling mark. Is it fair to a child to bring them into the world - to have six months, maybe a year, before their life spirals irreversibly and irrevocably down the drain to death?
No treatment. No treatments on the horizon.
What about anencephaly? A small percentage of these children (who have no brain at all due to a defect in formation) survive in utero - to be born, vegetables without brains, with only the most rudimentary of functions left in the brainstem, with a gaping hole in a misshapen head, eyes that will never see. They survive no more than a few days with the best of care...and you can't give someone a brain, nor can you reverse the deformations of development.