The mottled variety is extremely unique. A mottled budgie is hatched looking like a normal budgie. With each progressive molt, more and more of the budgie's feathers grow back clear. The budgie starts to look somewhat like a pied only with a more random, mottled pattern of clear feathers than the established varieties of pied. The amount of mottling an individual budgie has varies. Some have more normally marked and colored feathers than clear ones. Others eventually become almost all clear.

Basic Genetics:
Unknown/undetermined

For information on mottled, see The Mottle: Variety or Trait by Inte Onsman

photos courtesy Rinus van den Broek
The mottled variety increasingly develops clear feathers with each molt.