Difference Between Codominance and Incomplete Dominance

Main Difference – Codominance vs Incomplete Dominance

According to Mendel, the phenotype of the offspring always resembles the parents. On that account, the phenotype of the offspring is determined by the number of dominant and recessive alleles for a particular trait. But, the interactions between different alleles determining a particular trait complicate the Mendelian inheritance. The inheritance of alleles, which are not completely dominant or recessive, gives rise to the two concepts called codominance and incomplete dominance. The main difference between codominance and incomplete dominance is that in codominance, a cross between individuals with two different phenotypes produces an offspring with a third phenotype, showing both parental phenotypes together whereas, in incomplete dominance, a cross between individuals with two different phenotypes produces an offspring with a third phenotype, which is a blend of the two parental phenotypes. 

Key Areas Covered

1. What is Codominance
      – Definition, Features, Examples
2. What is Incomplete Dominance
      – Definition, Features, Examples
3. What are the Similarities Between Codominance and Incomplete Dominance
      – Outline of Common Features
4. What is the Difference Between Codominance and Incomplete Dominance
      – Comparison of Key Differences     

Key Terms: Codominance, Incomplete Dominance, Dominant Alleles, Mendelian Inheritance, Offspring, Parents, Phenotype, Recessive Alleles

What is Codominance

Codominance is a concept in which heterozygous offspring produces both alleles simultaneously without any mixing of the two parental alleles. In codominance, both parental alleles are dominantly expressed in the offspring. Both parental alleles can be observed in the offspring without blending. Thus, codominance is a qualitative approach of gene expression. Codominance mostly occurs when more than two alleles are present for the determination of the phenotype of a particular trait. Those alleles are called multiple alleles.

Figure 1: Hybrid Red and White Camellia

The roan cow containing both red and white hairs is an example of codominance. The AB blood group also shows codominance in humans. A cross between the red homozygous Camellia flowers and white homozygous Camellia flowers produces an offspring with both red and white spots within the same flower is shown in figure 1.

What is Incomplete Dominance

Incomplete dominance is a concept in which a heterozygous offspring produces a third trait with blended characters of the two parental traits. In incomplete dominance, both the parental alleles are expressed dominantly, but in different proportions. Therefore, an intermediate phenotype of both parental phenotypes can be observed in the offspring. Thus, incomplete dominance is a quantitative perspective of the two parental alleles.

Figure 2: Incomplete Dominance

The pink snapdragon is an example of incomplete dominance, which is produced by a cross between red snapdragon and white snapdragon. The Punnett square illustrating the incomplete dominance of pink snapdragon is shown in figure 2. 

Similarities Between Codominance and Incomplete Dominance

• Codominance and incomplete dominance are two mechanisms whose patterns of inheritance cannot be explained by the laws of Mendelian inheritance.
• Both codominance and incomplete dominance are caused by intragenic (allelic) gene interactions.
• Both codominance and incomplete dominance occur during the inheritance of a pair of heterozygous alleles.
• In codominance and incomplete dominance, both parental alleles inherited are dominant.
• In both mechanisms, the two parental phenotypes are involved in producing the third phenotype.

Difference Between Codominance and Incomplete Dominance

Definition

Codominance: Codominance is the appearance of both parental phenotypes together in the offspring when a cross is done between individuals with two different phenotypes.

Incomplete Dominance: Incomplete dominance is the appearance of a third phenotype, which is a combination of both parental alleles when a cross is done between individuals with two different phenotypes.      

Conspicuousness of Alleles

Codominance: In codominance, the effect of both alleles are equally conspicuous.

Incomplete Dominance: In incomplete dominance, the effect of one of the two parental alleles is more conspicuous.

Mixing

Codominance: In codominance, both parental phenotypes are mixed together to form a third phenotype.

Incomplete Codominance: In incomplete dominance, the two parental alleles are not mixed together.

Effect of Parental Alleles in the Phenotype

Codominance: In codominance, both parental alleles produce their effect independently.

Incomplete Dominance: In incomplete dominance, the effect of the two parental alleles is intermediate on the offspring.

Expression of Parental Alleles

Codominance: In codominance, both parental alleles can be observed in the offspring.

Incomplete Dominance: In incomplete dominance, neither of the parental alleles can be observed in the offspring.

Quantitative/Qualitative Approach

Codominance: Codominance is a qualitative approach of the gene expression.

Incomplete Dominance: Incomplete dominance is a quantitative approach of both incompletely dominant alleles.

Examples

Codominance: The roan character of cows and the inheritance of the AB blood group are examples of codominance.

Incomplete Dominance: The pink snapdragon, the inheritance of height, weight, eye color, and skin color are examples of incomplete dominance.

Conclusion

Codominance and incomplete dominance are two types of non-Mendelian inheritance patterns. Codominance is a quantitative perspective of parental alleles while incomplete dominance is a quantitative perspective. In codominance, both parental alleles of a particular trait may be equally expressed in the offspring. In incomplete dominance, a combination of parental alleles of a particular trait may be expressed in the offspring. The main difference between codominance and incomplete dominance is the mechanism of expressing parental traits within the offspring.

Reference:

1. Scoville, Heather. “Codominance – Evolution Definition.” ThoughtCo. N.p., n.d. Web. Available here. 12 July 2017. 
2. Bailey, Regina. “What Is Incomplete Dominance in Genetics?” ThoughtCo. N.p., n.d. Web. Available here. 12 July 2017. 

Image Courtesy:

1. “Co-dominance Rhododendron” By darwin cruz – Flickr (CC BY 2.0) via Commons Wikimedia
“Incomplete dominance” By Spencerbaron – Own work (CC BY-SA 3.0) via Commons Wikimedia