Okay, I did read some more and it sounds just like the frizzled gene or the short leg gene for Japanese chickens.
I still stand by my statement though that it's not abuse to breed these animals as long as you know how to properly cross them
It's not just a lethal gene. Crests on ducks are not formed by a different skeletal structure like with chickens. Crests are formed by holes in the ducks skull. Fatty tissue covers the hole, leaving a raised patch of
just tissue--no skull underneath. The feathers grow longer because of the raised tissue, forming a crest.
Because of the hole in their skull, they are incredibly prone to brain trauma. Getting hit over the crest can kill them because their brain will get hit. When drakes mount crested hens, they pull the crest, and the crest can get pulled off, leaving exposed brain.
These ducks also suffer neurological issues, including seizures. What if a duck has a seizure while swimming? She could drown!
Breeding these ducks is not only an issue because the ducklings can die in the shell. It is cruel to bring them into existence and have them suffer from the crest defect just because they are cute.
ETA: And most people don't even know this about crested ducks. They just buy them because they are cute, not knowing these ducks are very special needs. As a result, many suffer from the problems above. Even if someone did know these things about crested ducks, that does not prevent neurological problems, or accidental head injury.