While all breeds of chickens with mottled varieties are simple called "mottled," research has shown that there are actually at least four allelic mutations of the mottled (mo) gene.
Solid White from mottled (mo^w)
In recent years, some breeders have noticed an infrequent but consistent appearance of solid white birds from mottled breeding in certain breeds in the US, namely mottled Silkies, Satins, and black mottled D'uccle.
Research from 2014 and 2017 identified at least four different mutations of recessive mottling, including one (mo^w) that produces a solid white bird.
There is some anecdotal evidence from multiple breeders of mottled Silkie and Satin breeds that some solid white mottles (mo^w) birds appear to have very poor vision as well.
The Science
Excerpt from post Sep 10, 2019 by nicalandia on BackyardChickens.com:
Observations on Mottling and its many allelic mutations
Due to recent genetic mapping of the mottling allele (Endothelin Receptor B2 or EDNRB2 ) research have been able to identify at least four allelic mutations of the recessive mottling gene.
- Research work by Japanese team: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0086361 responsible for two mutation confirmation(mo^w and mo^j)
- Research work by Blacksburg, Virginia Team: https://vtechworks.lib.vt.edu/bitstream/handle/10919/85397/Li_J_D_2017.pdf responsible for two mutations(mo/mo and mo^d/mo^d)
mo^d/mo^d = Breed: Mille Fleur and Black Mottled D'uccle, the research found that no mutation was present and it was identical to the wildtype Mo+ allele, but they went ahead and speculated that it may be a different mutation on the EDNRB2 but on a non-coding region. non-coding DNA is often incorrectly referred to as junk DNA (no official designation has been given to the mutation so I propose mo^d/mo^d for D'uccle)
In genetics, there is always two different key aspects: how something looks (phenotype), and the genes that are at play (genotype). Sometimes things that look the same are genetically quite diverse. As research and evidence in mottled breeds advances, we are sure to learn more about our beloved speckled chicken friends!