Here are some definitions from a genetics textbook*:
Codominant -- expression of heterozygous phenotype resulting in hybrid offspring that resemble both parents equally for a particular trait.
Incomplete dominance -- expression of heterozygous phenotype resulting in offspring whose phenotype is intermediate between those of the parents.
By these definitions morphs like the pastel in ball pythons are not really codominant morphs by correct definition. They would be incomplete dominant morphs. A het of a truely codominant morph would show the traits of both the parents simultaneously as shown in part A of the figure below using coat pattern in lentils as an example.
The pastel trait in ball pythons is actually an incomplete dominant trait like coloration in snapdragons (part B of figure below) where the traits of the het are somewhere between those of the homozygous parents. So with pastel ball pythons the homozygous parents would be a normal (NN), and the super pastel (PP). A regular pastel would be the het (PN). A regular pastel is intermediate between the super pastel and a normal ball python. It does not exhibit both traits (normal coloration and pastel coloration) of the homozygous parents like the het of a codominant trait would.
So, if you could breed an all white ball python to a normal colored ball python (that is homozygous for normal at the pied locus) and get piebalds (which we all know you can't, but just to explain the point further) then this would be an example of a codominant trait. This is because a pied would show patches of all white (trait of one homozygous parent) and patches of normal coloration (trait of the other homozygous parent). I hope this makes sense. Knowing this probably won't change the habits already in the community, but I thought atleast some of you out there would be interested to know of this misnomer.
Ben
*Hartwell et. al. 2004. Genetics: from Genes to Genomes. 2nd edition. McGraw-Hill. New York.
