At the level we are working at, codominant and incomplete dominant can be used interchangeably. (I prefer codominant because it has fewer letters to type.) Codominant is definitely used too loosely, mostly because many herpers have a wrong idea of the definition.

Nature is sloppy, so you get shades of gray rather than black and white. Humans make the black and white distinctions. Then we shoehorn Nature's results into the best fit (not necessarily a perfect fit) with our definitions.

Here's the ideal range of appearances with recessive mutant genes (I'd like to make the space in the middle bigger but can't.):
|--| |--|
The right |--| is for creatures with two recessive mutant genes, and the left is for creatures with two normal genes and creatures with a normal gene paired with a recessive mutant gene.

Here's the ideal range of appearances with dominant mutant genes:
|--| |--|
The right |--| is for creatures with two normal genes. The left is for creatures with two dominant mutant genes and creatures with a normal gene paired with a dominant mutant gene. This is the mirror image of the arrangement for the recessive mutant gene.

Here's the ideal range of appearances with codominance:
|--| |--| |--|
with the |--| on the left the range of appearances for a creature with two mutant genes, the middle for a creature with a mutant gene paired with a normal gene, and the |--| on the right for a creature with two normal genes.

Sometimes Nature is sloppy and gives an overlap:
|--|---|---| |--|
The |--| is the creature with two normal genes.
The |--|----|---| contains both creatures with two mutant genes and creatures wih a mutant gene paired with a normal gene. The salmon mutant gene in the boa constrictor produces this sort of overlap. It's not a perfect fit with our definition of either dominant or codominant. In such cases, I consider that calling the mutant a dominant mutant gene is a better fit than calling it a codominant mutant gene.

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