codominance vs incomplete dominance

2 min read 18-03-2025

In the world of genetics, understanding how traits are inherited can be complex. While Mendelian genetics provides a basic framework, many inheritance patterns deviate from simple dominance. Two common examples are codominance and incomplete dominance, often confused but distinctly different. This article will clarify the differences between codominance and incomplete dominance, using clear examples to illustrate each concept.

What is Incomplete Dominance?

Incomplete dominance occurs when neither allele is completely dominant over the other. The heterozygote displays an intermediate phenotype—a blend of the two homozygous phenotypes. Think of it as a mixing of traits rather than one overpowering the other.

Examples of Incomplete Dominance

Flower color in snapdragons: A red snapdragon (RR) crossed with a white snapdragon (rr) produces pink offspring (Rr). The pink color is a blend of red and white, showing incomplete dominance.
Coat color in horses: A chestnut horse (RR) and a cremello horse (rr) can produce a palomino horse (Rr) with a coat color intermediate between chestnut and cremello.
Wave pattern of hair: A person with straight hair (SS) and a person with curly hair (CC) may have wavy hair (SC) demonstrating incomplete dominance.

Image showing incomplete dominance in snapdragons: red, pink, and white flowers

What is Codominance?

Codominance, in contrast, is when both alleles are fully expressed in the heterozygote. Neither allele masks the other; instead, both contribute to the phenotype. It's like having two distinct traits simultaneously visible.

Examples of Codominance

ABO blood groups: The A and B alleles are codominant. Individuals with the genotype AB have both A and B antigens on their red blood cells, expressing both alleles.
Coat color in cattle: A red bull (RR) and a white cow (WW) can produce roan offspring (RW). Roan cattle have a coat with both red and white hairs, showcasing both alleles.
Sickle cell anemia: Individuals heterozygous for the sickle cell trait (Hb^AHb^S) have both normal and sickle-shaped red blood cells. Both alleles are expressed, demonstrating codominance.

Image showing codominance in roan cattle: red and white hairs

Key Differences Between Incomplete Dominance and Codominance

The core distinction lies in how the alleles interact:

Feature	Incomplete Dominance	Codominance
Allele Interaction	Neither allele is completely dominant; heterozygote shows a blended phenotype.	Both alleles are fully expressed; heterozygote shows both phenotypes.
Phenotype	Intermediate phenotype (blend)	Both parental phenotypes are present
Example	Pink snapdragons from red and white parents	Roan cattle from red and white parents

How to Tell the Difference

Distinguishing between incomplete dominance and codominance relies on carefully observing the phenotype of the heterozygote. If the heterozygote displays a blend of parental traits, it's incomplete dominance. If the heterozygote expresses both parental traits simultaneously, without blending, it's codominance.

Beyond Simple Inheritance

Incomplete dominance and codominance demonstrate that inheritance patterns are more intricate than simple Mendelian genetics suggest. These non-Mendelian inheritance patterns highlight the complexity and diversity of genetic expression. Understanding these variations is crucial for a comprehensive understanding of heredity and its impact on phenotypic traits.

Further Exploration

For deeper dives into these concepts, explore resources from reputable genetics websites and textbooks. Understanding the nuances of these inheritance patterns will strengthen your understanding of genetics as a whole.