Incomplete Dominance and Codominance

[Yuqing]

I need some help on what exactly is happening here at the cellular level.

I understand that incomplete dominance is a blending, for example caused by a functional enzyme producing pigments and a non-functioning enzyme. The result would be less pigments, therefore an apparent mixing.

But what is happening with co-dominance? You have checkered chickens and roan cows. How exactly is this caused at a cellular level? If both phenotypes are active simultaneously, shouldn't there be a uniform blending instead of having patches of colour or similar phenomena?

 

[phi29]

I'm not quite sure of the physiology of coat colors, but consider blood type. The alleles are equally expressed on the surface of RBCs, they cannot physically blend.
Your patching question makes me think of Barr body inactivation, so that only X chromosome is activated in each cell and an almost chimerism can be seen, in that some only paternal X-chrom genes are expressed in a given cell, and maternal genes in another.
Hope this helped a little bit, genetics can be confusing since there are so many mechanisms that accomplish the same effect, especially in intro courses where molecular things aren't explained.