This crested gene is linked to a lethal condition during incubation. Ducklings carrying both genes for the crest do not survive to hatch. Of those that do hatch, typically 1/3 of them will not have crests.
I am guessing that the black and white duck on the right is also a mallard hybrid, because of his body shape and tightly curled tail feathers.
This photo was taken at a farm north of Port Colborne, Ontario. They had the weirdest collection of ducks and geese I have ever seen.