Here's how we used the terms when I worked in the Ia State Univ Genetics Lab and took the ISU introduction to genetics course.



Both Sean and Jeff are right. The normal gene is the baseline, and a mutant can be compared to the baseline as soon as the mutant is found. Two mutant alleles can only be compared after the second one is found, which could be tomorrow, ten years down the line, or never. So "codominant" either implies or specifies a comparison to another allele. If none is specified, the comparison is always to the normal allele. But either the normal allele or another mutant allele can be specified. "Pastel is a codominant" means the comparison is to the normal allele. "Pastel is codominant to the normal allele" specifies the comparison is to the normal allele. "Amelanistic is codominant to ultra" specifies the comparison is to a second mutant allele.



There is a tendency to restrict dominant, codominant, and recessive to ONLY the relation between a mutant and its wild-type (normal) allele. A two mutant heterozygote like ultramel would simply be described. For example, Ultra and amelanistic are recessive mutant genes. The ultra//amelanistic heterozygote is more or less intermediate in color between the ultra homozygote and the amelanistic homozygote. That last sentence is a lot longer and less convenient than "Amelanistic is codominant to ultra." In my opinion, the convenience will make people retain the "amelanistic is codominant to ultra" useage.



For simplicity, codominant and incomplete dominant can be used as synonyms. If you really want to get technical, the concept can be subdivided into at least three and probably more terms. Of course, that requires knowing something about the biochemistry. And we know very little about the biochemistry of reptile mutants. Many people on these forums have little knowledge of genetics, so I go with simplicity.



The Punnett square is a good learning tool but has drawbacks for heavy duty use. The pro geneticists I worked for hardly ever used the square. They did one gene pair problems in their heads and used the branching system (AKA decision tree, forkline) for two or more gene pairs. It gives the same answer, is quicker and is less prone to mistakes. It is a little less intuitive than the Punnett square, though. Unfortunately, I haven't found a good description on the web of a branching system applied to a genetics problem.



Paul Hollander

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