Side by side
| Incomplete dominance | Codominance | |
|---|---|---|
| What the heterozygote looks like | An intermediate phenotype, between the two homozygotes | Both alleles expressed simultaneously and equally, not blended |
| Standard textbook example | Snapdragon flower color: C^R C^R red, C^W C^W white, C^R C^W pink | Human MN blood groups: L^M L^M is M, L^N L^N is N, L^M L^N expresses both antigens |
| Genotypic ratio from a heterozygote self-cross | 1:2:1, the same as any Mendelian monohybrid cross | 1:2:1, the same as any Mendelian monohybrid cross |
| Phenotypic ratio from a heterozygote self-cross | 1:2:1, because the intermediate is its own visible class | 1:2:1, because all three genotypes are phenotypically distinct |
| Is either allele dominant | No; this is a gene interaction in which neither allele is completely dominant | No; neither masks the other, and the heterozygote shows both |
| Notation convention | Superscripted letters such as C^R and C^W, to signal a non-Mendelian pattern | Superscripted letters such as L^M and L^N, for the same reason |
| Relationship between the two | The base case, where expression appears intermediate | Described by OpenStax as a variation on incomplete dominance |
Two ways a heterozygote can refuse to hide an allele
In simple Mendelian dominance a heterozygote looks exactly like the homozygous dominant parent, which is why the F2 of a monohybrid cross collapses from a 1:2:1 genotypic ratio to a 3:1 phenotypic one. Both of these patterns break that collapse, but for different reasons. Under incomplete dominance the heterozygote expresses an intermediate phenotype, so a red parent and a white parent give pink offspring and neither allele is completely dominant over the other. Under codominance both alleles are expressed at the same time in the heterozygote, so the M and N antigens both appear on the red blood cell surface rather than averaging into something new.
A worked example
Two pink snapdragons (C^R C^W) are crossed. Give the genotypic and phenotypic ratios of the offspring, and the percentage expected in each flower color.
- Identify the pattern. Pink is the heterozygote of a gene showing incomplete dominance, so both parents are C^R C^W and each makes two gamete types, C^R and C^W, in equal numbers.
- Draw the 2 by 2 Punnett square. The four equally likely boxes are C^R C^R, C^R C^W, C^W C^R and C^W C^W.
- Collect the genotypes: 1 C^R C^R, 2 C^R C^W and 1 C^W C^W, which is the 1:2:1 genotypic ratio any monohybrid cross gives.
- Translate genotype to phenotype. Because the heterozygote is intermediate rather than hidden, each genotype is its own visible class: 1 red, 2 pink, 1 white. The phenotypic ratio is also 1:2:1.
- Convert to percentages out of the four boxes: 1/4 red is 25 percent, 2/4 pink is 50 percent, and 1/4 white is 25 percent. The three add to 100 percent, which confirms nothing was double counted.
= Genotypes 1 C^R C^R : 2 C^R C^W : 1 C^W C^W, phenotypes 1 red : 2 pink : 1 white, which is 25 percent red, 50 percent pink and 25 percent white.
Where students go wrong
The trap is treating incomplete dominance as evidence for blending inheritance, the pre-Mendelian idea that parental traits mix permanently. They do not. Cross two pink snapdragons and red and white reappear in the next generation, which is exactly what blending cannot explain and what a Punnett square predicts without effort. The second trap is assuming the underlying arithmetic changes. It does not. Both patterns keep the 1:2:1 genotypic ratio of any monohybrid cross; what changes is only how many phenotype classes those three genotypes produce, which is three here rather than the two that simple dominance gives.
Where this leads next
Both of these are inheritance patterns rather than new machinery, so the natural next stop is the genotype and phenotype page, where the distinction between an organism's genetic makeup and its observable traits is the whole point. From there, multiple alleles are the obvious extension: a gene can have more than two alleles in a population, as the four coat color alleles at the rabbit c gene do, and dominance hierarchies then appear between them. The Punnett square page covers the grid these ratios come out of, if the 1:2:1 counting is not yet automatic.
Common mistakes
- Calling pink snapdragons a blend of red and white. Cross two pinks and red and white reappear, which is what makes the alleles particulate rather than mixed.
- Expecting a 3:1 phenotypic ratio. With three visible classes the phenotypic ratio matches the 1:2:1 genotypic ratio instead.
- Using lower case and upper case letters for these crosses. OpenStax uses superscripted notation such as C^R and C^W precisely to flag that this is not simple dominance.
- Describing codominance as an intermediate phenotype. An MN individual is not halfway between M and N; both antigens are present at once.
Related concepts
- Dihybrid cross: A dihybrid cross follows two characteristics at once, starting from true-breeding parents that differ in both.
- Punnett square: A Punnett square applies the rules of probability to predict the outcomes of a genetic cross.
- Genotype vs phenotype: A genotype is an organism's underlying genetic makeup, including alleles that are not expressed.