From what I've found, the difference between "incomplete dominance" and "codominance" (both used in the strict sense) has to do with functionality of the gene products. In incomplete dominance, one gene has a nonfunctional gene product and the other gene has a functional gene product. In codominance, both genes have functional gene products.

With many mutant genes, we have no idea of the functionality of the gene products. In both cases, on the whole animal level, the heterozygote can be distinguished from both homozygotes. On that level, the two terms are synonyms. As "codominance" is the shorter term, I generally use it in preference to "incomplete dominance".

Paul Hollander

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