example of incomplete dominance

2 min read 19-03-2025

Incomplete dominance is a fascinating inheritance pattern where neither allele for a specific gene completely masks the other. Instead, the heterozygote displays a phenotype that's a blend or intermediate between the two homozygous phenotypes. This contrasts with complete dominance, where one allele completely overshadows the other. This article will explore incomplete dominance with clear examples and explanations.

Understanding Incomplete Dominance

In complete dominance, one allele (the dominant allele) completely masks the expression of another allele (the recessive allele). For instance, if 'B' represents the allele for brown eyes and 'b' represents the allele for blue eyes, a person with the genotype Bb will have brown eyes because 'B' is dominant over 'b'.

However, in incomplete dominance, neither allele is completely dominant. The heterozygote exhibits a third phenotype, a combination of the two homozygous phenotypes. If we use the same example but with incomplete dominance, a Bb individual might have hazel eyes – a blend of brown and blue.

Key Characteristics of Incomplete Dominance

Intermediate Phenotype: The most obvious characteristic is the heterozygote displaying a phenotype intermediate between the two homozygotes.
No True Recessiveness: Neither allele is truly recessive; both contribute to the phenotype in the heterozygote.
Blending of Traits: The resulting phenotype appears as a blend or mixture of the parental traits.

Examples of Incomplete Dominance

Let's explore some real-world examples to solidify our understanding.

1. Flower Color in Snapdragon Plants

One of the classic examples of incomplete dominance is flower color in snapdragons.

Homozygous Red (RR): Plants with two red alleles (RR) produce red flowers.
Homozygous White (rr): Plants with two white alleles (rr) produce white flowers.
Heterozygous Pink (Rr): Plants with one red and one white allele (Rr) produce pink flowers – an intermediate phenotype.

This clearly demonstrates the blending of traits. The pink flowers aren't simply a mixture of red and white patches; the entire flower is a uniform pink color.

2. Hair Color in Humans

While human hair color is complex and influenced by multiple genes, incomplete dominance can play a role.

Let's simplify and say:

Homozygous Dark Hair (DD): Individuals with two dark hair alleles have dark hair.
Homozygous Blonde Hair (dd): Individuals with two blonde hair alleles have blonde hair.
Heterozygous Light Brown Hair (Dd): Individuals with one dark and one blonde hair allele have light brown hair – a blending of the two.

Again, this is a simplification. Human hair color is a polygenic trait, meaning multiple genes contribute to its expression. However, incomplete dominance can contribute to the variation we observe.

3. Palomino Horses

Palomino horses offer another striking example.

Homozygous Chestnut (CC): Horses with two chestnut alleles have a rich chestnut coat.
Homozygous Cream (cc): Horses with two cream alleles have a creamy white coat.
Heterozygous Palomino (Cc): Horses with one chestnut and one cream allele have a distinctive golden coat with a flaxen mane and tail.

The palomino color is a clear intermediate between chestnut and cream.

Distinguishing Incomplete Dominance from Other Inheritance Patterns

It's crucial to differentiate incomplete dominance from other inheritance patterns like codominance. In codominance, both alleles are fully expressed in the heterozygote, resulting in a phenotype that shows both traits simultaneously (e.g., AB blood type). In incomplete dominance, there is a blending of traits resulting in a new phenotype.

Conclusion

Incomplete dominance provides a fascinating insight into the complexity of gene expression. It showcases that not all alleles behave in a simple dominant-recessive manner. By understanding incomplete dominance, we gain a deeper appreciation for the diverse ways genes interact and contribute to phenotypic variation in the natural world. Examples such as snapdragon flower color, human hair color (simplified), and Palomino horses clearly illustrate this nuanced genetic interaction.