Incomplete Dominance & Codominance

Beyond Mendel's clean 3:1

Gregor Mendel built classical genetics on seven pea traits — round versus wrinkled seeds, tall versus short stems, purple versus white flowers — that all behaved the same way. In every case, crossing two pure-breeding lines gave a first generation (F1) that looked exactly like one parent, and a second generation (F2) that split into a tidy 3:1 ratio of the dominant to the recessive phenotype. The dominant allele completely hid the recessive one in heterozygotes. This is complete dominance, and it is so clean that it can make inheritance seem almost digital.

It is also, biologically, a special case. When you look past peas to snapdragons, four-o'clock plants, cattle coats, and human blood, the heterozygote frequently does not resemble either parent. Sometimes it lands halfway between them, and sometimes it shows both parental traits at the same time. These two outcomes are incomplete dominance and codominance. They are not exceptions to Mendel's laws of segregation and independent assortment — the alleles still separate cleanly into gametes and recombine at fertilization in exactly the proportions Mendel predicted. What changes is only the genotype-to-phenotype map: how the molecular product of each allele translates into something you can see.

Incomplete dominance: the intermediate blend

The textbook case is the snapdragon (Antirrhinum majus) — or the four-o'clock, Mirabilis jalapa, which Carl Correns used in 1900. Cross a true-breeding red-flowered plant (genotype CRCR) with a true-breeding white one (CWCW) and every F1 plant is pink (CRCW). Neither red nor white appears in that generation. The pink is a genuine intermediate, not a patchwork: under a microscope the petal cells are uniformly pale pink, not a mosaic of red and white cells.

The mechanism is a dosage effect. The CR allele encodes a functional enzyme in the anthocyanin pigment pathway; the CW allele is a loss-of-function variant that makes little or no working enzyme. A homozygote with two functional copies makes a full dose of red pigment. A heterozygote with one functional copy makes only about half as much — too little to register as red, enough to register as pink. Geneticists call this haploinsufficiency: a single working copy of the gene is insufficient to produce the full wild-type phenotype. Whether a gene shows complete or incomplete dominance often comes down to whether 50% of the normal enzyme output is enough to do the whole job, or only half of it.

Self-pollinate the pink F1 and the F2 emerges in a 1 red : 2 pink : 1 white ratio. Because each of the three genotypes produces its own distinct, visible phenotype, the phenotypic ratio is identical to the genotypic ratio. This is the single most useful diagnostic for incomplete dominance: a 1:2:1 split instead of 3:1, with the heterozygotes immediately recognizable on sight.

Codominance: both, fully, at once

Codominance looks superficially similar — the heterozygote again differs from both parents — but the molecular story is opposite. Here both alleles encode functional, detectable products, and both are expressed fully and independently in the same individual. There is no blending and no dosage compromise; you simply see both phenotypes side by side.

The clearest visible example is coat color in cattle and horses. Cross a red-coated cow (CRCR) with a white-coated one (CWCW) and the offspring are roan: a coat carrying a dense mixture of fully red hairs and fully white hairs. From a distance roan can look pinkish, but up close each individual hair is unambiguously red or white. Compare this with the snapdragon, where each petal cell is itself pink. Blending versus mixing is the heart of the distinction.

The most consequential human example is the ABO blood group. A single gene specifies a glycosyltransferase enzyme that decorates red-blood-cell surface sugars. The IA allele adds N-acetylgalactosamine (the A antigen); the IB allele adds galactose (the B antigen); the recessive i allele makes a non-functional enzyme that adds neither. Someone with genotype IAIB makes both enzymes, so their cells display both A and B antigens — blood type AB. The two functional alleles are codominant with each other. Note the layered logic: IA and IB are each completely dominant over recessive i (so type-A blood can be IAIA or IAi), but codominant with each other. ABO is therefore a double example: multiple alleles (three at one locus) plus codominance.

Incomplete dominance vs. codominance vs. complete dominance

Feature	Complete dominance	Incomplete dominance	Codominance
Heterozygote phenotype	Identical to dominant homozygote	Intermediate blend of the two	Both parental phenotypes shown together
Molecular basis	One allele's product suffices alone	Dosage / haploinsufficiency (~50% product)	Both alleles make distinct, detectable products
F2 phenotypic ratio	3 : 1	1 : 2 : 1	1 : 2 : 1
Phenotype = genotype?	No (3 genotypes → 2 phenotypes)	Yes (3 genotypes → 3 phenotypes)	Yes (3 genotypes → 3 phenotypes)
Classic example	Mendel's pea flower color	Snapdragon / four-o'clock (red×white→pink)	ABO blood (AB); roan cattle; sickle trait

Notice that incomplete dominance and codominance share the same 1:2:1 ratio. You cannot tell them apart from the numbers alone — only by inspecting the heterozygote. Ask one question: is the heterozygote a blend (pink) or a mixture/both-at-once (roan, AB)? Blend means incomplete; both-at-once means codominant.

Why it matters clinically: sickle cell and beyond

The genotype-to-phenotype map is not fixed even within one gene — it depends on which level you measure. Sickle-cell disease is the textbook demonstration. The HbS allele carries a single base change (a Glu→Val substitution at the sixth amino acid of β-globin). A heterozygote (HbA/HbS) produces both hemoglobins — roughly 60% normal hemoglobin A and 40% sickle hemoglobin S — so at the protein level the alleles are codominant. But at the clinical level the trait looks recessive: one functional allele makes enough normal hemoglobin to keep red cells healthy under ordinary oxygen conditions, so carriers are largely symptom-free. The same alleles read as codominant, recessive, or even (under low oxygen, when carriers can sickle) something in between, depending on whether you look at the molecule, the cell, or the whole person.

This carrier state is also why the sickle allele persists in malaria-endemic regions, where carrier (heterozygote) frequencies can reach 25–40% and the allele frequency itself climbs to around 10–15%: HbA/HbS heterozygotes are substantially protected against severe Plasmodium falciparum malaria, a heterozygote advantage that balances the lethality of the HbS/HbS homozygote. Familial hypercholesterolemia behaves similarly — heterozygotes for the LDL-receptor mutation have intermediate cholesterol and intermediate heart-disease risk, while homozygotes are far more severely affected, a clear incomplete-dominance dose response with direct prognostic weight. In transfusion medicine, recognizing ABO codominance is literally life-saving: an AB recipient tolerates A, B, AB, and O blood, while giving the wrong type triggers fatal hemolysis.

Key points to keep straight

• Same ratio, different mechanism. Both give 1:2:1 in F2; the heterozygote's appearance (blend vs. both) is what separates them.
• Mendel still holds. Segregation and independent assortment are untouched — only the visible expression of the heterozygote differs.
• Dominance is about phenotype, not the gene itself. The same alleles can look codominant, recessive, or incompletely dominant depending on which level you measure.
• Codominance ≠ incomplete dominance ≠ mixed inheritance. Pink petals are blended pink cells; roan coats are separate red and white hairs.
• Multiple alleles can layer in. ABO combines three alleles with both complete dominance (over i) and codominance (A with B).