You may have done this, but i would research the internet regarding this….somewhere in my mind i remember reading that some cases can be traced to diet or how they are hatched….am not saying that it couldn't be hereditary too,
After checking a few sites, it could be hereditary, but also said incubatory humidity too low or riboflavin or vit. deficiency.
I am not saying it isn't possible but these birds eat the same as my others, are hatched the same etc. I would think I would be seeing it with my others as well. I am stumped.