Quote Originally Posted by Russ Lawson View Post
It appears to me that the only textbook example I've been shown of co-dominance that differed in any way from incomplete dominance (being blood type) is actually the result of two dominant mutant alleles at the same locus. I believe the AB heterozygous individual is technically the only thing that can be correctly called co-dominant if anything in the mix was to be because both the A and B phenotype are equally expressed.

Just compare it to making a mojave x lesser BEL. Say mojave is AO and lesser is BO (O = wild-type). AB is a mojave x lesser BEL - when bred to a normal, all you get is AO and BO. In the case of human blood type though, AA and AO (or BB and BO) are phenotypically identical, unlike in the BEL complex.

The reason that it appears this way is not because it is true, but because of the nature of blood typing.

When we talk about 'A,' 'B,' 'AB,' and 'O,' we are talking about specific markers on the surface of the cell. 'O' is the absence of markers. That is why, phenotypically, 'AA' and 'AO' are the same thing- you gain nothing from the O. Both are being expressed equally, but you just can't see the 'O' being expressed.

Also, a roan cow (or horse or whatever) is not the same as incomplete dominance. You are thinking of the animal as a whole, not the individual hairs. Take a strawberry roan- they have both red and white hairs. Some hairs are red, some are white. If this color trait were incompletely dominant instead of codominant, you would have an animal with hairs that were all one color, and that color would be an intermediate between the two- so a lighter shade of red.

Think of it as flowers. If you take a white flower and crossbreed to a red one, a plant with incomplete dominance for the color gene will produce a pink flower. A plant that has a codominant color gene will produce a flower with some white petals and some red petals.