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"Fixed" is often used loosely here - sure you can put feathers on a clean shanked bird's offspring in one generation - but it will not fix your future offspring. You will continue to throw clean shanks for several generations until you figure out which of your birds is throwing the clean shanks. Clean shanks are recessive, so the genes can hide in feather shanked birds. There are also suspected recessive inhibitors that affect shank feathering as well.
If it were that easy, there'd be a lot more feather shanked Cuckoos running around out there.
Feathering is supposed to be dominant, but the heterozygous expresses in different ways. You might even have a bird with full leg/toe feathering that is heterozygous. Bottom line - if you don't want to keep seeing clean shanks in your flock for a long time, don't breed ANY clean shanked birds. If you don't care, and are just experimenting, then have fun, and let us know what your stats end up being. I suspect it's harder than expected to put consistent feathers on shanks when introducing clean shanks to your flock... like less than 50% will have them.