Note that for recessive traits (such as Tay Sachs and Cystic fibrosis), genetic testing which is widely down in the frum community can prevent a shidduch in which both parents have the trait (which is what is required for a possibility of having a child with the condition), and that even if both parents are carriers, there is only a 25% chance that any given child will have the trace. Due to genetic testing (and arranged marriages), these diseases are increasingly rare in our community. While it is possible, the odds of having 7 out of 10 children display a recessive genetic trait are quite small (it can happen, but is extremely unlikely).
It should also be noted that Cystic fibrosis is treatable, and most people with the disease live until middle age, and in the US, most graduate from university. I’ld question whether killing a kid for having the disease is worth discussing even if allowed. Downs syndrome results in a variety of developmental disabilities, but kids with the disease (which is not a genetic disease) often live to adult hood. The boys get bar mitzvas. They often can be self-supporting with adult but do need family support. Tay Sachs is currently incurable, but that is liable to change. Any psak halacha from more than a few years ago should be considered meaningless since conditions are changing, and the psak should come from a rav who is keeping up on current medical developments.