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Co dominance is most often expressed on the cell level and not in gross morphology ( feathers,etc). For example in henny feathering. With henny feathering- the henny feathering gene is turned on in males and produces an enzyme called aromatase- this protein converts testosterone (male hormone) to estradiol (estrogen or female hormone). If a male is heterozygous (has only one henny featheing gene) for henny feathering he produces 1/2 the amount of aromatase in his skin as a male bird that has two henny feathering genes. So you do have co dominance in cellular activity but not on the gross morphology level. A male with one henny feathering gene or two henny feathering genes has henny feathering so the gene is considered dominant in males.
Pea comb is supposed to be incompletely dominant but from my working with heterozygous pea combed fowl it is the only thing I can think of that appears to be codominant. The top of a male comb looks like a large pea comb and the bottom of the comb appears to be a single comb, same thing on females. So both genes are being expressed.
Tim
Tim, I love it when you explain the cellular level workings of genes! There is so very little of that information out there (that I've found), and you do an incredible job of explaining it.
I have to say though, that your description of the henny feathering sounds more like incomplete dominance than co-dominance.
I, too, had thought that comb genes were as close as I could come to co-dominance. For example, the spikes and protuberances on some rose combs, the trifid spikes that can occur on silkies, the various combinations of comb genes and how they express (walnut, split single, buttercup, walnut + duplex, etc.)