Short Answer
When Mendel first described inheritance, he focused on traits that showed a clear dominant‑recessive relationship. Subsequent research revealed many exceptions, two of the most common being incomplete dominance and codominance. Both involve heterozygous individuals that do not simply look like the dominant homozygote, but the way the phenotype is produced is fundamentally different. This article defines each pattern, contrasts them across several dimensions, and provides concrete examples and a worked cross so you can apply the concepts.
Learning Objectives
- Define incomplete dominance and codominance.
- Identify the phenotypic outcome of a heterozygote for each pattern.
- Compare the two patterns using a structured table.
- Predict offspring ratios for a classic incomplete‑dominance cross (snapdragons) and a codominance cross (human blood type).
- Recognize common misconceptions and avoid them in future analyses.
What Is Incomplete Dominance?
In incomplete dominance the phenotype of the heterozygote is a blend of the two homozygous phenotypes; neither allele completely masks the other. The classic example is the flower colour of snapdragons (Antirrhinum majus). The allele R produces red pigment, while W produces no pigment (white). A plant with genotype RR is red, WW is white, and the heterozygote RW is pink – an intermediate colour that is not a patchwork of red and white but a uniform blend.1
What Is Codominance?
Codominance occurs when both alleles in a heterozygote are fully expressed, giving a phenotype that displays features of each allele side by side. A textbook example is the human ABO blood‑type system. The IA allele encodes an enzyme that adds N‑acetylgalactosamine to the H antigen, while IB adds galactose. Individuals with genotype IAIB express both A and B antigens on the surface of red blood cells, resulting in the AB blood type – a clear coexistence of both traits rather than a blend.3
Side‑by‑Side Comparison
| Dimension | Incomplete Dominance | Codominance |
|---|---|---|
| Heterozygote phenotype | Intermediate or blended (e.g., pink flower) | Both parental phenotypes expressed together (e.g., AB blood type) |
| Allele interaction | Partial expression of each allele; neither fully dominant | Each allele produces a distinct product that is independently functional |
| Typical molecular basis | Reduced amount of functional product from each allele (e.g., half the pigment enzyme) | Separate functional products coexist (e.g., two different enzymes encoded by each allele) |
| Classic visual example | Snapdragon flower colour (R = red, W = white, RW = pink) | Human ABO blood type (IA = A antigen, IB = B antigen, IAIB = AB) |
| Punnett square phenotype ratios (heterozygote × heterozygote) | 1 : 2 : 1 (e.g., red : pink : white) | 1 : 2 : 1 (e.g., type A : AB : type B) for a cross of IAIB × IAIB |
| Common in plants vs. animals | Frequent in flower colour, fruit size, and other quantitative traits | Common in blood groups, coat patterns (e.g., roan cattle), and some flower colour spots |
Both patterns belong to the broader category of non‑Mendelian inheritance, which also includes multiple alleles, sex‑linked traits, and epistasis2.
Why the Distinction Matters
Correctly identifying whether a trait follows incomplete dominance or codominance influences genetic predictions, breeding strategies, and medical counseling. For example, in horticulture a breeder aiming for a particular shade of flower must understand that crossing two pink snapdragons (RW × RW) will still produce a 1:2:1 ratio of red, pink, and white, not a uniform pink offspring. In medicine, recognizing that the AB blood type is codominant ensures accurate blood‑type matching for transfusions.
Worked Example: Predicting Offspring
Example 1 – Incomplete Dominance (Snapdragons)
Cross two heterozygous pink snapdragons (genotype RW × RW).
- Set up a Punnett square with each parent’s gametes (R and W).
- Fill the square:
- Top‑left:
RR→ red - Top‑right:
RW→ pink - Bottom‑left:
WR→ pink (same asRW) - Bottom‑right:
WW→ white - Count phenotypes: 1 red, 2 pink, 1 white → 1 : 2 : 1 ratio.
This demonstrates how the intermediate phenotype persists in the heterozygote generation.
Example 2 – Codominance (Human Blood Type)
Cross two individuals with genotype IAIB (AB blood type) with each other.
- Each parent can produce two types of gametes:
IAorIB. - Punnett square yields:
IAIA→ type AIAIB→ type ABIBIA→ type AB (same as above)IBIB→ type B- Phenotypic ratio: 1 : 2 : 1 (A : AB : B).
Note that the AB phenotype appears twice because both alleles are expressed simultaneously, rather than blending into a new colour.
Common Misconceptions
Self‑Check Questions
- Which inheritance pattern best describes the coat of a cattle animal that shows both white and red patches simultaneously?
Answer: Codominance (white and red hairs are both expressed). - In a plant where red flower colour is dominant over white, a heterozygote shows pink flowers. Is this an example of incomplete dominance or codominance?
Answer: Incomplete dominance, because the phenotype is an intermediate blend. - When two AB‑type humans have a child, what is the probability that the child will have blood type O?
Answer: 0 % – neither parent carries the recessiveiallele required for type O.
Next Learning Step
After mastering incomplete dominance and codominance, explore multiple‑allele inheritance and how epistatic interactions modify phenotypic ratios in more complex genetic networks.
FAQ
What is the main difference between incomplete dominance and codominance?
In incomplete dominance the heterozygote shows an intermediate, blended phenotype (e.g., pink snapdragon flowers). In codominance both alleles are fully expressed, producing a phenotype that displays both parental traits side by side (e.g., AB blood type).
Can you give a classic plant example of incomplete dominance?
The flower colour of snapdragons (Antirrhinum majus) is a textbook case. Red (RR) × white (WW) crosses produce pink (RW) heterozygotes, which are intermediate between the two parental colours.
What human trait illustrates codominance?
The ABO blood‑type system is codominant. Individuals with genotype IAIB express both A and B antigens on red blood cells, resulting in the AB blood type.
What phenotypic ratios are expected from a heterozygote × heterozygote cross for each pattern?
Both incomplete dominance (RW × RW) and codominance (IAIB × IAIB) produce a 1 : 2 : 1 ratio of the three possible phenotypes. For incomplete dominance this is red : pink : white; for codominance it is type A : AB : type B.
Why is it important to distinguish between these two inheritance patterns?
Correct identification influences genetic predictions, breeding strategies, and medical decisions. For example, knowing that snapdragon colour follows incomplete dominance helps breeders anticipate the 1:2:1 ratio, while recognizing AB blood type as codominant ensures accurate transfusion matching.
Are there any common misconceptions about codominance and incomplete dominance?
Yes. A frequent error is thinking codominance is just a form of incomplete dominance; in reality codominance shows both traits fully, whereas incomplete dominance blends them. Another myth is that a pink snapdragon is a mosaic of red and white patches; it is actually a uniform colour due to reduced pigment production.
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