I had mottled houdans in the past and the mottling varied from bird to bird. Some birds showed a little mottling and others showed large patches of white feathers. It is also my understanding that as the bird ages more mottling will show in the plumage.
From Carefoots work with mottling and the pied (exchequer) phenotypes, there may be more than one allele at the mottling locus. I also believe that the columbian gene will modify the mottling phenotype restricting the white to the tip of the feather.
If you compare the mottling in a speckled sussex with black/mottled birds there is a big difference in the two types of mottling. The ancona is an example of a black that has white restriction.
If the columbian gene is causing the difference, a cross between a mottled orpngton and a rhode island red should reveal some interesting offspring results.
Then crossing the offspring should produce some black birds with mottling that is restricted to the tips of the feathers and not an unorganized mottled phenotype.
Edited by Wappoke - 5/24/16 at 3:50pm