Have you seen this thread, Don? https://www.backyardchickens.com/t/579569/leg-color-in-black-copper-marans
From the French Marans web site: http://www.marans.eu/genetiqa.htm#genetiqa (reference to the white fluff /feathers appears at the end)
GENETIC CHARACTERISTICS OF SHANK COLOURATION
The skin colouration of poultry (for example, white skin or yellow skin) and the dermis of the shanks can be independently affected by the presence of specific genes responsible for leg colour in poultry.
There are indeed two different genes expressed by the signs (id+) and (ID) which are responsible for the presence or absence of “melanin” (black pigments) in the skin of the shanks.
The gene (id+) imposes the appearance of melanin in the dermal layer of the shanks, which then shows a bluish gray color in chickens with white skin (in the case of the white Bresse, for example).
However, found in the genotype of a breed with yellow skin, the shanks then appear olive-green (black breasted red English Game and certain Northern Games).
This genetic darkening characteristic of the legs (id+) is a recessive sex-linked trait.
The dominant gene (ID) causes, on the contrary, the absence of melanin on the level of the shanks, which are then expressed as rosy-white in the breeds with white skin (Marans or Gâtinaise, for example) or yellow in the breeds with yellow skin (Northern Games, Wyandottes).
On this subject, let us benefit from the occasion to recall that certain genes for plumage colour like “cuckoo” (B), “wheaten” (Ewh), or “splash” (Bl/Bl) are accompanied by a hereditary inhibition of the melanin, which explains in particular why all poultry breeds with “cuckoo” and “wheaten” plumage as in the Marans, have invariably clear shanks beaks and nails which are of course genetically dependent for their colour.
Nevertheless, for the Marans varieties with dominant black genes such as the self-coloured black, Silvered Black [Birchen], and Black-copper [Brown-red] the presence clear shanks involves an additional difficulty in the selection of breeding stock. In effect, this trait often appears in conjunction with the presence of light or white down or undercolour along with the appearance of unwanted white feathers at the base of the tail or wheaten flight feathers if they are not totally white.
