>The difference between incomplete and codominance is a little trickier. Incomplete dominance is easier to understand, giving a blending of the two phenotypes. When genes are codominant, it's not blending exactly, but both specific phenotypes are seen in the same animal. An example is blood type. A and B are codominant to each other and dominant to O. This means that each cell produces A and B antigens, not a single blended version.



Unfortunately, this is not on the same scale. Blood typing is on a cellular or even molecular scale, while "blending of the two phenotypes" is on a whole animal scale. Looked at on the molecular scale, in both incomplete dominance and codominance the two genes do their own thing, producing a mixture of two gene products rather than blending the two into a single product.



Best definition distinguishing incomplete dominance from codominance I've seen is this: Incomplete dominance -- one allele produces a functional enzyme, and the other allele produces a nonfunctional enzyme.

Codominance -- Both alleles produce functional enzymes.



In both codominance and incomplete dominance, there are three phenotypes -- one for the first homozygote, one for the second homozygote, and one for the heterozygote. When we lack enzyme functionality information and just use phenotype information, incomplete dominance and codominance are synonyms.



Paul Hollander

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