definition for incomplete dominance

2 min read 19-03-2025

Incomplete dominance is a pattern of inheritance where neither allele for a specific trait is completely dominant over the other. This results in a heterozygous phenotype that's a blend or intermediate between the two homozygous phenotypes. Unlike complete dominance, where one allele masks the other completely, incomplete dominance shows a third, unique phenotype. Let's delve deeper into this fascinating genetic concept.

Understanding Alleles and Phenotypes

Before we explore incomplete dominance, it's crucial to understand some basic genetic terms:

Alleles: Different versions of the same gene. For instance, a gene for flower color might have one allele for red and another for white.
Genotype: An organism's genetic makeup, representing the combination of alleles it possesses. For example, RR (red), rr (white), or Rr (a combination of red and white alleles).
Phenotype: An organism's observable characteristics, which are determined by its genotype and environmental influences. This is what you actually see – the color of the flower, for example.

In complete dominance, one allele (the dominant allele) completely masks the expression of the other allele (the recessive allele). However, in incomplete dominance, neither allele is completely dominant.

Examples of Incomplete Dominance

Several examples illustrate the principle of incomplete dominance:

Flower Color in Snapdragon Plants

One classic example is the flower color in snapdragon plants.

RR: Produces red flowers.
rr: Produces white flowers.
Rr: Produces pink flowers.

The heterozygous Rr plants exhibit a pink phenotype, an intermediate between the red and white parental phenotypes. This demonstrates incomplete dominance, as neither the red nor the white allele is completely dominant.

Hair Texture in Humans

Human hair texture also shows traits consistent with incomplete dominance in some populations.

CC: Curly hair
cc: Straight hair
Cc: Wavy hair

Individuals with heterozygous Cc genotype possess wavy hair, a mixture of curly and straight hair.

Coat Color in Andalusian Chickens

Andalusian chickens offer another striking example.

BB: Black feathers
bb: White feathers
Bb: Blue feathers

The blue-feathered chickens demonstrate incomplete dominance. Their phenotype is a blend of black and white, resulting in blue.

How Incomplete Dominance Differs from Complete Dominance and Codominance

It's important to differentiate incomplete dominance from other inheritance patterns:

Complete Dominance: One allele completely masks the expression of the other. The heterozygote exhibits the phenotype of the dominant allele.
Codominance: Both alleles are fully expressed in the heterozygote. Neither allele masks the other; instead, both contribute to the phenotype. A classic example is the AB blood type in humans, where both A and B antigens are present.

Incomplete dominance represents a distinct inheritance pattern where the heterozygous phenotype is a blend of the homozygous phenotypes, offering a unique expression.

Conclusion: The Nuances of Inheritance

Incomplete dominance highlights the complexity of inheritance patterns. It's crucial to remember that while these examples illustrate clear intermediate phenotypes, the degree of blending can vary depending on the specific genes and environmental factors. Understanding incomplete dominance adds another layer of depth to our comprehension of genetics and heredity.