Incomplete Dominance

Codominance And Incomplete Dominance
Figure 6-9 Red, pink, and white phenotypes of four o'clock plants. The pink heterozygote demonstrates incomplete dominance. [R. Calentine/Visuals Unlimited.]

of the intermediate phenotype suggests an incomplete dominance, the term used to describe the general case in which the phenotype of a heterozygote is intermediate between those of the two homozygotes, on some quantitative scale of measurement.

How do we explain incomplete dominance at the molecular level? In cases of incomplete dominance, each wild-type allele generally produces a set dose of its protein product. The number of doses of a wild-type allele determines the concentration of some chemical made by the protein (such as pigment). Two doses produce the most copies of transcript, hence the greatest amount of protein, and hence the greatest amount of chemical. One dose produces less chemical, and a zero dose produces none.


The human ABO blood groups are determined by three alleles of one gene. These three alleles interact in several ways to produce the four blood types of the ABO system. The three major alleles are i, IA, and IB, but one person can have only two of the three alleles or two copies of one of them. The combinations result in six different genotypes: the three homozygotes and three different types of heterozygotes.

Genotype Blood type

Genotype Blood type


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