example of incomplete dominance

2 min read 20-03-2025

Incomplete dominance is a fascinating inheritance pattern where neither allele for a particular gene completely masks the other. This results in a heterozygous phenotype that's a blend of the two homozygous phenotypes. Unlike complete dominance, where one allele completely overshadows the other, incomplete dominance shows a unique, intermediate trait in the offspring. Let's explore this with examples.

Understanding Incomplete Dominance

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

However, in incomplete dominance, neither allele is completely dominant. If we use the same example, but assuming incomplete dominance, a person with the genotype "Bb" wouldn't have brown eyes. Instead, they might have hazel eyes – a blend of brown and blue.

This blending of traits is the hallmark of incomplete dominance. It demonstrates that gene expression isn't always a simple "on" or "off" switch.

Examples of Incomplete Dominance

Several examples illustrate incomplete dominance across various species:

1. Flower Color in Snapdragons

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

Homozygous Red (RR): Produces red flowers.
Homozygous White (rr): Produces white flowers.
Heterozygous (Rr): Produces pink flowers.

The pink flowers in the heterozygous offspring demonstrate the blending of the red and white alleles. Neither allele is completely dominant, resulting in an intermediate phenotype.

2. Coat Color in Shorthorn Cattle

Shorthorn cattle exhibit a similar pattern of incomplete dominance in their coat color:

Homozygous Red (RR): Produces red coat.
Homozygous White (rr): Produces white coat.
Heterozygous (Rr): Produces roan coat (a mix of red and white hairs).

Again, the heterozygote shows an intermediate phenotype, a blend of the parental traits.

3. Human Hair Curliness

Human hair texture is another example where incomplete dominance might play a role, although it's more complex than simple RR, rr, Rr inheritance.

Curly Hair (CC): Represents individuals with tightly curled hair.
Straight Hair (cc): Represents individuals with completely straight hair.
Wavy Hair (Cc): Represents individuals with wavy hair.

While the inheritance of hair curliness is influenced by multiple genes, incomplete dominance provides a simplified model to understand the concept. Many other genes and environmental factors also contribute to hair texture.

4. Familial Hypercholesterolemia (FH)

This genetic disorder demonstrates incomplete dominance in a more medically significant way. FH involves high cholesterol levels.

Homozygous Dominant (HH): Individuals typically experience severe hypercholesterolemia.
Homozygous Recessive (hh): Individuals have normal cholesterol levels.
Heterozygous (Hh): Individuals exhibit moderately elevated cholesterol levels.

This example shows how incomplete dominance can lead to varying degrees of disease severity depending on the genotype.

Why Does Incomplete Dominance Occur?

Incomplete dominance occurs because neither allele completely suppresses the expression of the other at the molecular level. The resulting protein product from the heterozygote often has reduced function compared to the homozygous dominant genotype. This leads to a noticeable intermediate phenotype.

Distinguishing Incomplete Dominance from Other Inheritance Patterns

It's crucial to distinguish incomplete dominance from other patterns like codominance. In codominance, both alleles are fully expressed in the heterozygote, rather than blended. A good example is the AB blood type in humans, where both A and B antigens are present.

Conclusion

Incomplete dominance highlights the complexity of gene interaction and phenotypic expression. By understanding this pattern, we gain a deeper appreciation for how genes influence traits and contribute to the diversity of life. Studying these examples helps us better understand the underlying mechanisms of heredity and their implications for various organisms, including ourselves.