Codominance and incomplete dominance both describe what happens when neither allele in a pair is fully dominant, but they produce visibly different results. In incomplete dominance, the two alleles blend into an intermediate phenotype. In codominance, both alleles are fully expressed at the same time, side by side. The difference comes down to blending versus coexisting.
What Happens in Incomplete Dominance
Incomplete dominance produces a heterozygous phenotype that falls somewhere between the two homozygous parents. The classic example is snapdragon flower color. When you cross a red-flowered snapdragon with a white-flowered one, all the offspring come out pink. Neither the red allele nor the white allele wins out completely, so you get something in the middle.
At the molecular level, this happens because only one of the two alleles is producing pigment. That single working allele makes enough pigment to turn the flowers pink, but not enough to make them fully red. It’s essentially a dosage effect: one copy of the pigment-producing allele gives you half the color intensity. This is different from complete dominance (like in pea plants), where a single copy of the dominant allele produces enough protein to generate the full trait.
What Happens in Codominance
In codominance, both alleles are expressed simultaneously and fully, rather than blending together. You don’t get an intermediate. Instead, both traits show up in the organism at the same time.
The most familiar example is the ABO blood type system. A person who inherits one A allele and one B allele doesn’t end up with some blended “AB-ish” blood. Instead, their red blood cells display both the A sugar and the B sugar on their surface. Both alleles independently produce their respective proteins, and both proteins are present and detectable. The result is type AB blood.
Another striking example is roan coat color in cattle. When a red-coated cow is crossed with a white-coated one, the offspring don’t have a uniform pink coat. Instead, they have a mix of individual red hairs and individual white hairs growing side by side. Each hair is entirely one color or the other. From a distance, the coat looks like a blended roan color, but up close, you can see both parental traits expressed independently.
The Core Distinction: Blending vs. Both
The simplest way to tell the two apart is to look closely at the heterozygous offspring. If you see a new, intermediate trait that neither parent had (pink flowers from red and white parents), that’s incomplete dominance. If you can identify both parental traits present simultaneously in the same organism (red hairs and white hairs on the same cow, or A and B sugars on the same blood cell), that’s codominance.
- Incomplete dominance: The heterozygote looks like a blend. Red + white = pink.
- Codominance: The heterozygote shows both traits. Red + white = red and white patches or markers.
Both Share the Same Genetic Ratios
Despite looking different at the phenotype level, codominance and incomplete dominance produce identical genotypic ratios when you cross two heterozygous individuals. In both cases, the offspring follow a 1:2:1 pattern. For snapdragons, that’s 1 red, 2 pink, 1 white. For the MN blood group system (a codominant trait), that’s 1 type M, 2 type MN, 1 type N.
This is a key difference from standard Mendelian dominance, where a cross between two heterozygotes yields a 3:1 phenotypic ratio because the heterozygote looks the same as the homozygous dominant. In both codominance and incomplete dominance, you can distinguish the heterozygote by looking at it, so the phenotypic ratio matches the genotypic ratio.
Some Traits Show Both Patterns at Once
Certain genetic conditions blur the line between codominance and incomplete dominance depending on how closely you look. Sickle cell disease is the best example. A person with one normal hemoglobin allele and one sickle cell allele (sickle cell trait) produces both normal and sickle-shaped red blood cells. At the molecular level, that’s codominance: both types of hemoglobin protein are made, and both are present in the blood.
But zoom out to the whole-body level, and the picture looks more like incomplete dominance. People with sickle cell trait generally have milder symptoms than someone with two sickle cell alleles. Their overall health phenotype falls between unaffected and fully affected, which is the hallmark of an intermediate, blended outcome. Whether you call it codominance or incomplete dominance depends on the scale of observation, which is why genetics textbooks sometimes classify sickle cell trait as either one.
How to Tell Them Apart in Practice
If you’re working through a genetics problem or trying to classify a real trait, ask one question: can you see both parental phenotypes in the heterozygote, or do you see something new? If a flower is pink and neither parent was pink, that’s a new intermediate phenotype, pointing to incomplete dominance. If a blood test shows both A and B antigens, those are two parental traits coexisting, pointing to codominance.
The underlying genotypic math is the same for both. The difference is entirely about what happens when those gene products show up in the body: do they mix into something new, or do they each do their own thing?