Incomplete dominance – Definition, Example, Mechanism, Ratios

Incomplete dominance is a genetic phenomenon where neither allele in a gene pair completely masks the other. This results in a phenotype that is a blend of both parental traits. Commonly observed in characteristics like flower color and human skin tone, incomplete dominance showcases the complex interactions of genes beyond traditional dominant-recessive patterns. This concept is crucial for understanding the diversity of genetic inheritance in both natural and controlled breeding contexts.

Incomplete Dominance Definition

Incomplete dominance is a genetic phenomenon where the phenotype (observable characteristics) of the offspring is a blend of the phenotypes of the parents. This occurs because neither allele (gene variant) in the gene pair completely masks the other. The result is a new, intermediate phenotype that is not seen in either parent.

After Gregor Mendel established the foundational laws of inheritance, Carl Correns, a German botanist, introduced the concept of “incomplete dominance.” He explored this genetic principle through experiments with four o’clock flowers. His findings revealed that in some cases, heterozygous individuals exhibit a phenotype that is a blend of both dominant and recessive traits, rather than expressing just the dominant trait. This discovery demonstrated that inheritance could involve more complex interactions than Mendel’s original models suggested.

Incomplete Dominance Example

For example, if you cross a red-flowered plant with a white-flowered plant, incomplete dominance results in offspring with pink flowers. The pink color is neither completely red nor completely white, showing that no allele dominates the other. Instead, both alleles express themselves, leading to an intermediate phenotype. This blending effect marks incomplete dominance as distinct from complete dominance, where one allele completely masks the other.

Explaining Incomplete Dominance Through Experiments

Mendel’s Pea Plant Experiments: Complete Dominance Gregor Mendel’s experiments on pea plants demonstrated complete dominance, where one allele completely masks the presence of another. For instance, when Mendel crossed peas with round seeds (dominant) and wrinkled seeds (recessive), all first-generation (F1) offspring exhibited round seeds, showcasing the dominance of the round seed trait.

Transition to Incomplete Dominance However, not all traits follow Mendel’s pattern of complete dominance. This was further explored through Carl Correns’ experiments on four o’clock flowers, which vividly displayed incomplete dominance. Correns crossed plants with red flowers (dominant) and white flowers (recessive) and observed that the F1 generation did not show red or white flowers but pink ones, indicating no complete dominance by either allele.

Mechanism of Incomplete Dominance

Genetic Interaction In incomplete dominance, the alleles of a gene pair in a heterozygote express themselves fully without overshadowing each other. This results in a phenotype that is not typical of either parent but is rather a blend of both parental traits.

Biochemical Basis At the biochemical level, incomplete dominance typically involves the production of a diminished amount of a functional protein by the dominant allele. For instance, if an allele is supposed to produce a pigment at full capacity but instead produces only half, and the recessive allele produces none, the result is a phenotype that has half the normal pigment concentration.

Case Study: Snapdragon Flowers

Monohybrid Cross in Snapdragons Consider the classic example of the snapdragon (Antirrhinum sp.), where a cross between a true-breeding red flower (RR) and a white flower (rr) results in F1 progeny with pink flowers (Rr). This illustrates the incomplete dominance where the red and white traits blend to produce pink.

Self-Pollination of F1 Progeny Further self-pollination of the F1 progeny yields an F2 generation exhibiting red, pink, and white flowers in a 1:2:1 genotypic ratio. This genotypic ratio aligns with Mendel’s findings; however, the phenotypic ratio changes due to the blending of colors, demonstrating incomplete dominance.

How does Incomplete Dominance work?

• Allele Interaction: In cases of incomplete dominance, each allele in a heterozygous organism contributes to the phenotype, but neither can assert complete dominance. This interaction results in a phenotype that is a mix of both alleles.
• Protein Production: The alleles involved typically code for a specific protein. In incomplete dominance, the quantity or functionality of the protein produced by each allele is not sufficient to render the full trait. For example, if one allele produces half the normal amount of a pigment, and the other produces none, the resulting phenotype will have only half the typical pigment concentration.
• Phenotypic Outcome: The intermediate phenotype arises because the expression of each allele results in a partial phenotype. Continuing with the pigment example, this results in a lighter color than the one produced by the fully functioning allele but darker than that produced by the non-functional allele.
• Alleles and Phenotypes: In incomplete dominance, both alleles for a specific trait are expressed, resulting in a new, blended phenotype. For example, if a red-flowered plant is crossed with a white-flowered plant, the result may be pink flowers.
• Genotype vs. Phenotype: The genetic makeup (genotype) of organisms in incomplete dominance scenarios involves two different alleles. However, the observable characteristics (phenotype) are neither like one parent nor the other but are instead a mixture of both.
• F1 and F2 Generations: In the first generation (F1), all offspring typically show the intermediate phenotype. When these F1 individuals are crossed with each other (F2 generation), the resulting phenotypic ratio often follows a 1:2:1 pattern – one showing one parent’s phenotype, two showing the mixed phenotype, and one showing the other parent’s phenotype.

Incomplete Dominance in Humans

In the case of hair texture:

• Curly Hair Gene (C): Often considered a dominant allele.
• Straight Hair Gene (S): Typically a recessive allele.

When an individual inherits one curly hair allele (C) and one straight hair allele (S), the result is not purely curly or straight hair but rather a blend of both, leading to wavy hair. This intermediate phenotype, wavy hair (CS), demonstrates incomplete dominance as neither the curly nor the straight hair allele is completely dominant over the other.

Creating a Punnett Square for Snapdragon Flower Color

A Punnett square helps predict the outcome of a genetic cross. The alleles from one parent are listed on the top and those from the other parent on the side. Here’s how you set it up for a cross between red (RR) and white (WW) snapdragon plants:

List Possible Gametes

• Red parent (RR): All gametes will have the R allele.
• White parent (WW): All gametes will have the W allele.

Set Up the Punnett Square

Analyze the Results

• RW: All offspring (100%) from this cross will have one R allele and one W allele, making them all pink due to the incomplete dominance between R and W.

Genotypic and Phenotypic Ratios

In a typical monohybrid cross involving incomplete dominance, the genotypic and phenotypic ratios are often the same due to the direct observation of the intermediate phenotype in heterozygous individuals. Here’s how it generally breaks down in a cross between two heterozygous pink-flowered snapdragons (RW × RW):

• Genotypic Ratio:
  • 1 RR : 2 RW : 1 WW
• Phenotypic Ratio:
  • 1 Red : 2 Pink : 1 White

These ratios indicate that there is a 25% chance of producing red flowers, a 50% chance of producing pink flowers, and a 25% chance of producing white flowers in the F2 generation.

Incomplete Dominance with Multiple Alleles

The interaction of multiple alleles through incomplete dominance adds another layer of complexity to genetic inheritance. Here, instead of a simple dominant-recessive relationship, the alleles blend their traits in the phenotype of the heterozygote. This scenario can lead to a wide range of phenotypic expressions, even more varied than with simple incomplete dominance involving just two alleles.

Example: Coat Color in Rabbits

A classical example of multiple alleles interacting through incomplete dominance is the coat color in rabbits. The gene for coat color in rabbits has multiple alleles, including 𝐶C (full color), 𝑐𝑐ℎcch (chinchilla), 𝑐ℎch (Himalayan), and 𝑐c (albino). The 𝐶C allele is dominant over all other alleles, but 𝑐𝑐ℎcch, 𝑐ℎch, and 𝑐c show various degrees of incomplete dominance and interaction, resulting in different levels of pigment and patterns in the fur.

Educational Importance

For educators and students, understanding multiple alleles and incomplete dominance provides deep insights into genetic diversity and the mechanics of inheritance. This topic is essential for grasping more complex genetic concepts like polygenic traits and epistasis, which further explain the vast diversity observed in biological traits across different organisms.

FAQs

What is Incomplete Dominance?

Incomplete dominance occurs when two alleles produce an intermediate phenotype, neither completely dominant over the other.

What is Codominance and Incomplete Dominance?

Codominance involves both alleles being fully expressed, while incomplete dominance results in a blended phenotype.

What is an Example of Codominance?

A classic example of codominance is the AB blood type, where both A and B alleles are equally expressed.

Is Incomplete Dominance an Example of Blending?

Yes, incomplete dominance is an example of blending, where the offspring’s phenotype is a mix of both parents’ traits.

What are 3 Examples of Incomplete Dominance?

Three examples of incomplete dominance include Snapdragon flower coloration, Andalusian fowl plumage, and human hair texture.