Beyond Simple Dominance: Incomplete Dominance, Codominance and Multiple Alleles

When the 3 : 1 rule breaks

Cross a red snapdragon with a white one and, instead of red, you get pink flowers. Cross a red shorthorn bull with a white cow and the calf is roan: a speckled mix of red and white hairs. And the ABO blood types (A, B, AB and O) come from a gene with three alleles, not two.

Mendel's peas happened to show complete dominance. Many genes do not. This lesson adds three patterns to your toolkit. The good news: the Punnett square method stays exactly the same; only the way you read phenotypes changes.

Notation

With complete dominance we use A and a. When neither allele is dominant, a capital and a lower-case letter would be misleading, so we use a base letter with superscripts:

• C for the colour gene, with Cᴿ (red) and Cᵂ (white). The three genotypes are CᴿCᴿ, CᴿCᵂ, CᵂCᵂ.

Some textbooks write R and W, or R and R′. Any clear key is fine, as long as you define it.

Incomplete dominance

In incomplete dominance, the heterozygote has an intermediate phenotype. In snapdragons, CᴿCᴿ is red, CᵂCᵂ is white, and CᴿCᵂ is pink. One Cᴿ allele makes only about half as much red pigment as two do.

This is not blending inheritance. Cross two pink plants and red and white flowers reappear: the alleles were never mixed, just partly expressed.

Codominance

In codominance, both alleles are fully expressed in the heterozygote, side by side. Shorthorn cattle that are CᴿCᵂ have a roan coat: each individual hair is either fully red or fully white, and the two kinds are mixed. Look closely and you can see both parents' traits; nothing is in between.

Multiple alleles: ABO blood types

A gene can have more than two alleles in a population, although each person still carries only two (one on each homologue). The ABO blood-type gene has three common alleles:

• Iᴬ: makes A antigens on red blood cells
• Iᴮ: makes B antigens
• i: makes neither

Iᴬ and Iᴮ are codominant with each other, and both are completely dominant to i. That gives six genotypes and four phenotypes:

Your immune system makes antibodies against the ABO antigens you don't have. That is why blood for a transfusion must be matched: type O red blood cells have no A or B antigens, so they can be given to people of any ABO type (universal donor), and type AB people have no anti-A or anti-B antibodies, so they can receive red cells of any ABO type (universal recipient). A separate gene, the Rh factor (Rh-positive is dominant), must also be matched. Organizations such as Canadian Blood Services type every donation for ABO and Rh.

Other multiple-allele genes

In rabbits, a coat-colour gene has four alleles in a dominance series: full colour (C) > chinchilla (cᶜʰ) > Himalayan (cʰ) > albino (c). Each rabbit still has just two of them.

Many traits, such as human height and skin colour, are influenced by many genes (polygenic traits) and by the environment, so they show a smooth range rather than a few clear categories. Single-gene patterns are where genetics starts, not where it ends.