I was told it is the result of two different genes. Normal mallard, with stripes, is dominant, but both Khakis and WHs have a different recessive gene which cancels the stripes. They both still carry stripes at the other gene’s location though. When we crossed the breeds, they ended up heterozygous (one normal, one cancel for each breed) for normal stripes. The explaination fits but doesn’t match what Storey’s Guide to Ducks says. If the guide where right though, our ducklings wouldn’t have stripes.