Folks, as you know, I've been noodling on this issue of white feathers. Specifically, are the white underhackle and solid white wing and/or tail feathers caused by the same gene? Then, I began to wonder if the white feathers are somehow related to the shank color questions we've been discussing.
I sent another note to Roger Woo - WHAT a fountain of genetic knowledge that man is! Anyway, here is his reply - I have gotten his permission to post here. I don't know that it actually answers, conretely, anything any of us have asked about at this point, BUT - it's awesome information, and it DOES clear some things up for me, at least - I hope for others, it will do the same:
As I read about melanizers, I realize we really don’t know anything about how they work in combination with other genes. Melanizers will work one way on the birchen allele, and differently on the wheaten allele. They do different things in the presence of Columbian or Darkbrown, which can hide in BCM and barely express at all. Gender and hormones affect melanization dramatically as well.
So as to whether the pale shanks are connected to white feathers (whether the fluff or tails/wings) I don’t think anyone can say with conviction that it is connected in general in all marans.
All the roosters I have gotten from eggs in Germany have had pale shanks. Some have had white in wing or tail, some have not. I have not seen any correlation. I am not seeing restricted melanin in underfluff.
I have seen a lot of pictures of overmelanized Marans over the years. Even show winners. It is possible that because of the lack of the Id gene to clear the shanks to pale slate, that folks have bred toward the elimination of some of the melanizers when selecting for lighter shanks. This could have an affect on fluff or feathers being undermelanized, but it seems you would be throwing lots of mossy hens at the same time. Hens are usually more easily melanized than roosters though, so it could not have an affect on hens.
I think that Co does play a role in BCM somewhere. Its effects in conjunction with other genes is still not completely known. It may be what, in conjunction with some yet unknown melanizing genes (some maybe being recessive) that gives proper neck hackle color in hens, and perhaps causes a bit of copper in the chest on roosters. It’s possible that it has an affect on underfluff color as well. None of this has been researched or tested.
Some folks might have seen a correlation in a particular line or in a closed flock, but in general, in all the reading I have done on the Id gene that clears shanks, I’ve read nothing about light underfluff. Black Wyandottes are completely melanized partridge (same E allele as your SPPR’s) and they still have fairly clear yellow shanks. You can have pale shanks on a black bird with no affect on plumage if it is done the right way. Leghorns are melanized birchen with clear yellow shanks. Proves the same thing. Shank color and plumage are not locked together.