Hello!
I have a question: I've read about an mc1r allele called buttercup (x), but i can't quite confirm if this is what the breed sicilian buttercup has. Do you know if they really have that, or the name's just a coincidence?
The allele "e^bc" "Buttercup base" was discovered in the Buttercup breed and therefore named after the Buttercup breed, however, I don't believe it actually exists! Most folks that are into poultry genetics do not acknowledge e^bc. I believe Sicilian Buttercups are actually a modified Partridge but they looked just different enough that scientists originally assigned a different allele to them. I've never seen a picture of e^bc isolated from the pattern modifiers.
Genetic analysis could determine if it is a different gene. Surprisingly, a lot of the genes in chickens haven't actually been analyzed even though the chicken is the most numerous (and economically most important) bird in the world. But nobody cares about color. However, even if e^bc exists it doesn't matter much because whatever Buttercups have works just like Partridge.
Sicilian Buttercup chicks (from Feathersite.com)
Gold Pencilled Hamburg chicks (from Murray McMurray)
Gold pencilled Hamburg (just looks like a Buttercup that has been selected to have more black patterning. From Murray McMurray.)
Personally, I think they got excited realizing there are more than two E locus alleles and they just got carried away. A lot of the old research they did does seem a bit disorganized. Who can blame them? They were trying to figure things out!