Incomplete Dom versus Codom?

Ok, I apparently posted accidently while editing - here is the complete version of my answer. Sorry about that!

As has been said - don't let terminology used in herpetoculture influence your answers in biology! I'm sure that many of the traits that are called codominant in ball pythons, leopard geckos, corn snakes, etc are actually examples of incomplete dominance. Incomplete dominance and codominance are very similar in that they both produce visible hets. The difference is in how the various alleles are expressed.

Punnett squares work exactly the same way for simple dominant/recessive, codominant, and incompletely dominant traits. The difference is in the phenotype - the latter two have visible hets. So when considering a one-gene trait, you will get a 1:2:1 genotypic and phenotypic ratio with both codominant and incompletely dominant traits. The difference is only that we usually use two capitol letters R1 and R2 for example instead of R and r. (The 1 and 2 should be superscript - can't figure out how to do that here, though.)

Simple dominance example

R r
R RR Rr
r Rr rr

Genotypic ratio 1RR:2Rr:1rr
Phenotypic ratio 3 red : 1 pink

Incomplete dominance example (again, numbers should be superscript)

R1 R2
R1 R1R1 R1R2
R2 R1R2 R2R2

Genotypic ratio 1R1R1: 2R1R2: 1 R2R2
Phenotypic ratio 1 red: 2 pink: 1 white

I find it easiest to understand the difference between incomplete dominance and codominance by thinking about what the alleles are doing (protien synthesis). The job of any allele is to synthesize proteins - different versions of the allele may synthesize different versions of the protein, or it may fail to synthesize the normal protein.

With a simple dominant/recessive gene, the dominant allele suppress the recessive one. So, you have just the dominant allele synthesizing protein. The single dominant allele can produce enough protein to produce the full affect of the allele, so a heterozygous individual looks just like a homozygous dominant individual.

With Incomplete dominance, the more dominant allele still suppresses expression in the other less dominant allele. However, this time the more dominant one is not able to produce enough protein to produce the full effect of the allele. So, a heterozygous individual will look different than homozygous dominant (and homozygous recessive). In general, the heterozygotes will have an intermediate appearance.

In a codominant trait, neither allele is dominant over the other and both are expressed fully. Both alleles are able to produce enough protein for the full expression of the trait. In humans, we have three possible alleles to indicate blood type: A, B, and O (these are usually represented as IsuperscriptA, IsuperscriptB, and i). A and B are both dominant over O, but A and B are codominant. So, an individual that has both an A and a B allele has type AB blood and will have red blood cells with A antigens and B antigens on their surface.

I'm no genetics whizz, and I have certainly forgotten the most obvious differences between incomplete and true codominant genes, but in general I assume most of our morphs are incomplete dominants because the mixes almost always appear blended together like your pink flowers, instead of the spotted flowers.

I could most certainly be wrong about that, I find it's hard to determine if what double or triple animal you are seeing is truly an incomplete or codominant animal. :oops: (of course, I havent really sat down and tried to decide what they were and just refer to them as co-doms like everyone else :P)