Incomplete dominance and codominance are two important patterns of inheritance in genetics. They explain how certain traits or characteristics appear in offspring when parents carry different versions (alleles) of a gene. Although both involve interactions between alleles, they differ in the way these alleles influence each other’s expression. Below, let us explore these concepts clearly and simply.

What is Incomplete Dominance?

Incomplete dominance occurs when a dominant allele does not completely mask the effect of a recessive allele. As a result, the offspring shows a blend of the two parental traits. This is different from complete dominance, where one allele completely overshadows the other.

Incomplete Dominance Example:
A classic example is the cross between red-flowered (RR) and white-flowered (rr) snapdragons (also known as Antirrhinum majus). The heterozygous offspring (Rr) produce pink flowers, illustrating a blend of red and white traits.

Key Points:

Offspring display an intermediate phenotype (mixed appearance).
A new phenotype emerges in the hybrid generation.
It is different from complete dominance, where the dominant trait alone is visible in the heterozygous offspring.

What is Codominance?

In codominance, both alleles in a heterozygous genotype express themselves equally and separately. Neither allele can mask the other, so both traits are visible in the offspring without blending.

Example: The blood Group ABO system in humans is the most well-known example. The A allele and the B allele are both dominant when compared to the O allele, but they are codominant relative to each other. Hence, a person inheriting one A allele and one B allele (genotype AB) will have a blood group that shows both A and B antigens on their red blood cells.

Key Points:

Both parental traits are clearly expressed without blending.
No new phenotype is formed, but rather, a combined effect of both alleles is observed.

Incomplete Dominance vs Codominance: Main Differences

Feature	Incomplete Dominance	Codominance
Definition	The dominant allele does not completely mask the recessive allele	No allele can mask the other; both alleles are equally expressed
Phenotypic Expression	Displays a blended or intermediate trait	Both traits appear simultaneously, distinctly
Appearance of Offspring (Hybrids)	A new phenotype arises (e.g., pink flowers)	No new phenotype; the two parental phenotypes are visible side-by-side
Example	Pink flowers in snapdragons or four o’clock plants (Mirabilis jalapa)	Blood Group ABO, Roan coat colour in cattle (red and white hair appear together)
Comparison with Complete Dominance	Less dominant alleles can still affect the trait (partial dominance)	Both alleles share dominance without partial blending

Notice that in incomplete dominance vs codominance, the presence of an intermediate phenotype is a hallmark of incomplete dominance, whereas codominance allows each allele’s trait to be distinctly visible.

Difference Between Complete Dominance, Incomplete Dominance, and Codominance

Sometimes, it is helpful to compare all three inheritance patterns side by side:

Complete Dominance:

The dominant allele completely covers up the recessive allele.
The recessive trait remains unseen in heterozygotes.
Example: In peas, the purple flower colour is completely dominant over white.

Incomplete Dominance:

The dominant allele partially masks the recessive allele.
The offspring blend traits of both parents (pink flowers in snapdragons).

Codominance:

No allele hides the other.
The offspring show both traits separately (AB blood group).

Short Quiz (With Answers)

Try this quick quiz to strengthen your understanding:

Which inheritance pattern involves the blending of parental traits?
A. Codominance
B. Incomplete Dominance
C. Multiple Alleles
Answer: B. Incomplete Dominance

In codominance, which of the following statements is correct?
A. Both alleles are recessive
B. One allele is partially dominant
C. Both alleles are equally expressed.
Answer: C. Both alleles are equally expressed

Which example is associated with codominance?
A. Pink snapdragon flowers
B. AB Blood Group
C. White and red four o’clock plants
Answer: B. AB Blood Group

In incomplete dominance, a cross between a pure tall plant and a pure dwarf plant would produce
A. All tall plants
B. All dwarf plants
C. Plants of medium height
Answer: C. Plants of medium height

FAQs on Difference Between Incomplete Dominance and Codominance

1. Is incomplete dominance the same as partial dominance?

Yes, incomplete dominance is sometimes referred to as partial dominance because the dominant allele only partially masks the recessive allele’s effect, leading to an intermediate phenotype.

2. Can a single gene show both incomplete dominance and codominance simultaneously?

Usually, a single gene shows only one type of dominance pattern at a time. However, different genes in an organism may exhibit different patterns of inheritance (some could be codominant, others incomplete dominant).

3. Why is the ABO blood group often used as an example of codominance?

In the ABO blood group, the A and B alleles are both expressed in individuals with genotype AB, demonstrating clear codominance. Both antigens A and B appear on the red blood cells, with neither allele hiding the other.

4. Do incomplete dominance and codominance follow Mendel’s laws?

They generally follow Mendel’s Law of Segregation and Law of Independent Assortment, but they show exceptions to the way dominant and recessive alleles interact (as Mendel’s classic studies mainly dealt with traits that followed complete dominance).

5. How does incomplete dominance vs codominance impact breeding in plants and animals?

In plant or animal breeding, recognising whether a trait follows incomplete dominance or codominance helps breeders predict offspring appearance. This information guides selective breeding to produce specific desired traits, such as certain flower colours or coat patterns.