Originally Posted by Dipsy Doodle Doo
I bet you ARE thrilled, Gootziecat! One step at a time...
Kev, Tell me more about Id, please.
RIP Jailbird. She was so pretty.
Flower, Sorry about your guy, (I missed reading that he passed).
Pity two beautiful birds were lost.
Id is Inhibitor of Dermal melanin. It's a specific layer of skin, on leg areas of chickens this layer produces pigments- the default state as all wild junglefowls have dark colored legs. So Id is a mutant gene in domestic chickens and it happens to be a dominant sex linked one too.
Id afaik, only works by blocking pigmentation in this specific layer of skin.. it has no visible effects on feather color or on the pigmentation in other layers of skin. All chickens have either white or yellow skin, Id has no effect on this yellow or white skin because it's in a different layer of the skin. (black skin is really hyper- pigmentation of connective tissue-which is found throughout the body- rather than a "skin color", which is why the other organs and parts of the body becomes dark, not just the skin)
Okay so all chickens have yellow or white skin.. combine that with Id blocking the production of pigments in a specific layer of skin on the legs.. the result is the bird has either yellow or white legs, respectively.. Basically it's their body skin color showing through on the legs.
If a yellow skin bird does not have Id, that layer of skin will be able to make pigment and it will "combine" with the yellow body skin pigmenation to give visually green legs. I struggle to come up with a green legged breed but it is common in hatchery stock turkens, EE and backyard mixes.
If a white skin bird does not have Id, the white skin combined with the pigmented layer in legs will give visually blue/slate legs. Examples are Sebrights.
You can kind of think of it this way- Id prevents blue or green legs(and seriously messes with black skin pigmentation). It's pretty safe guess to assume all yellow or white legged birds have Id. Leghorns(yellow legs), Marans(white legs), RIR, Wyandottes(both yellow legs), it's definitely present in hatchery and backyard stock NN- it's not too hard to find yellow or white legged NN. Very common and widespread gene, it is no wonder the idea "FM(black skin) is sex linked" gets spread around, because the chances of someone crossing with a bird carrying Id is rather high.
Naturally, there's also more to the story.... Id is not very easily detectable on black and birchen chickens. It's because the gene that makes their feathers black also produce black pigments on their legs- in the layer Id has no/weak effect on.. It's due to this reason why it's very hard to make a good, solid black chicken with clear yellow or white legs- which is precisely the reason black legs is allowed on black Naked Neck standard when yellow legs is required on all other accepted colors & the same situation happens in other breeds too.
One way to find out a black(feathered) chicken with black legs has Id or not is by looking at soles, if the soles are distinctly yellow or white(again, the color depends on what body skin the chicken has) then it probably has Id. Some black feathered breeds call for specific color soles- probably means those breeds require Id, even though they may be otherwise solid black chickens like black/blue Orps, Jerseys, etc. So Id is much more common and widespread than it seems at a glance really.
This got a bit long but hope it's clearer than mud. :)