Why do some ducks that normally don't have mohawks get them?

Some ducks can have a genetically abnormal growth of the skull skin and/or feathers on their heads that is from previous introductions of any of the Crested breed of domestic duck (Swedish Crested Ducks are one of the more popular breeds) This can be passed on at a 50-50 ratio if the Crested breeds with a wild duck species such as a Mallard. The resulting offspring may have no feather pigmentation from the Crested and look like a pure Mallard and still have the mow-hawk/fluff ball on top of their heads.