The codominance myth

[CH2O2]

There are two genes, A and a. The A gene may be the wild type gene, and the a gene may be a mutant gene. Or the A gene may be a mutant gene, and the a gene may be the wild type gene. Or the A and a genes may be different mutant genes. The A gene and the a gene can form three gene pairs, AA, Aa, and aa.

Incomplete dominant genes:
1. Phenotype definition. The AA genotype produces the AA phenotype. The Aa genotype produces the Aa phenotype. And the aa genotype produces the aa phenotype. All three genotypes can be inferred from the phenotypes. AND the Aa phenotype is more or less intermediate between the AA and aa phenotypes.
2. Molecular genetics definition. The AA genotype produces the AA phenotype. The Aa genotype produces the Aa phenotype. And the aa genotype produces the aa phenotype. All three genotypes can be inferred from the phenotypes. AND one gene produces a functional enzyme or other product while the other gene produces a nonfunctional product.

Codominant genes (narrow classification):
1. Phenotype definition. The AA genotype produces the AA phenotype. The Aa genotype produces the Aa phenotype. And the aa genotype produces the aa phenotype. All three genotypes can be inferred from the phenotypes. AND the contribution of both the A gene and the a gene can be detected in the Aa phenotype.
2. Molecular genetics definition. The AA genotype produces the AA phenotype. The Aa genotype produces the Aa phenotype. And the aa genotype produces the aa phenotype. All three genotypes can be inferred from the phenotypes. AND both genes produce functional products.

Sources for the molecular genetics definitions:
Knight, Jeffrey A. and Robert McClenaghan. Encyclopedia of Genetics. Salem Press, Pasadena, California, USA, 1999. 2 vols.
Zubay, Geoffrey. 1987. Genetics. The Benjamin/Cummings Publishing Co., Menlo Park, CA. 1987, 973 pp. ISBN 0-8053-09100-3

As far as I know, the biochemistry of the pastel mutant gene has not been worked out. But here is one way it could work.

The pastel mutant gene could produce a functional product but a different amount of it than the normal gene (which also produces a functional product). Total the amounts produced from two normal genes, from a pastel and a normal gene, and two pastel genes. A pastel and a normal gene produce less than two normal genes, and the pastel royal is lighter than a normal. Two pastel genes produce the least, and the super pastel royal is lightest of all. That is the way the burmese and siamese genes work in cats. The homozygous burmese cat is the darkest, the homozygous siamese is the lightest, and the tonkinese cat (with a burmese gene paired with a siamese gene) is roughly intermediate between the other two.

I will not venture to speculate on motives. But it took my breath away the first time someone told me that the following definitions of homozygous and heterozygous were too complicated to learn.

Homozygous = the two genes in a gene pair are the same.
Heterozygous = the two genes in a gene pair are NOT the same.

If we use the codominante molecular definition and if the pastel gene is functional (at a lower degree compared to the normal) i would agree to call the morph codominant.

Excelent explanation by the way.