incomplete dominance vs codominance

2 min read 18-03-2025

Understanding how traits are inherited is fundamental to genetics. While Mendelian genetics provides a basic framework, many inheritance patterns deviate from simple dominance. Two such patterns are incomplete dominance and codominance, often causing confusion. This article will clarify the differences between incomplete dominance and codominance, illustrating them with examples.

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.

Examples of Incomplete Dominance:

Snapdragon flower color: A red snapdragon (RR) crossed with a white snapdragon (rr) produces pink snapdragons (Rr). The pink flowers are a mix of red and white, not a distinct third color.
Hair texture: In some populations, straight hair (HH) and curly hair (hh) alleles exhibit incomplete dominance. Individuals with the heterozygous genotype (Hh) have wavy hair, an intermediate between straight and curly.

What is Codominance?

In codominance, both alleles are fully expressed in the heterozygote. Unlike incomplete dominance, there's no blending; instead, both traits are clearly visible.

Examples of Codominance:

ABO blood groups: The alleles for the A and B blood group antigens are codominant. Individuals with the genotype AB have both A and B antigens on their red blood cells, exhibiting both phenotypes simultaneously.
Roan cattle: Roan cattle have a coat with both red and white hairs. This occurs because the alleles for red (R) and white (W) hair are codominant. Heterozygous individuals (RW) display both red and white hairs, not a pink blend.

Key Differences Between Incomplete Dominance and Codominance:

Feature	Incomplete Dominance	Codominance
Heterozygote	Shows an intermediate phenotype (blending of traits)	Shows both phenotypes simultaneously
Allele Expression	Neither allele is fully dominant	Both alleles are fully expressed
Phenotype	A blend of parental phenotypes	Distinct expression of both parental phenotypes
Example	Pink snapdragons from red and white parents	AB blood type from A and B blood type parents

How to Distinguish Between Incomplete Dominance and Codominance:

The key to distinguishing between these patterns lies in carefully observing the phenotype of the heterozygote. Is it a blend (incomplete dominance), or does it express both traits clearly (codominance)?

Analyzing the offspring of crosses between homozygous parents can further clarify the inheritance pattern. In incomplete dominance, the F1 generation displays a distinct intermediate phenotype, while in codominance, both parental phenotypes appear together in the F1 generation.

Beyond Simple Inheritance:

Incomplete dominance and codominance highlight the complexity of inheritance beyond simple Mendelian dominance. Many traits show more intricate patterns, influenced by multiple genes and environmental factors. Understanding these variations is crucial for comprehending the diversity of life and the mechanisms underlying genetic inheritance.