incomplete dominance definition biology

2 min read 20-03-2025

Incomplete dominance, also known as partial dominance, is a form of intermediate inheritance in which one allele for a specific trait is not completely dominant over the other allele. This results in a third phenotype, in which the expressed physical characteristic is a blend of the phenotypes of both alleles. Unlike complete dominance, where one allele masks the other completely, incomplete dominance produces an intermediate offspring. This is a key concept in Mendelian genetics.

Understanding Alleles and Phenotypes

Before diving deeper into incomplete dominance, let's review some fundamental genetics terms:

Alleles: Different versions of a gene. For example, a gene for flower color might have one allele for red and another for white.
Genotype: The genetic makeup of an organism, represented by the combination of alleles (e.g., RR, Rr, rr).
Phenotype: The observable physical characteristics of an organism, determined by its genotype (e.g., red flowers, pink flowers, white flowers).

How Incomplete Dominance Works

In incomplete dominance, neither allele is completely dominant. When a heterozygous individual (possessing two different alleles) expresses the trait, the resulting phenotype is a blend of the two homozygous phenotypes.

Let's use the example of snapdragons. If a red-flowered snapdragon (RR) is crossed with a white-flowered snapdragon (rr), the offspring (Rr) will have pink flowers. This is because the red allele (R) isn't fully dominant over the white allele (r). The result is a combination, a pink phenotype, demonstrating incomplete dominance.

Examples of Incomplete Dominance in Biology

Several examples illustrate incomplete dominance in various organisms:

Snapdragons: As mentioned above, the flower color in snapdragons shows a classic example of incomplete dominance.
Four O'Clock Plants: Similar to snapdragons, the flower color in four o'clock plants exhibits incomplete dominance. A cross between a red-flowered plant and a white-flowered plant will produce pink-flowered offspring.
Human Hair: In humans, the curly hair allele and straight hair allele sometimes demonstrate incomplete dominance. Individuals with one allele for curly hair and one for straight hair often have wavy hair, an intermediate phenotype.
Palomino Horses: The coat color of Palomino horses is a result of incomplete dominance. A cross between a chestnut horse and a cremello horse can produce a Palomino horse with a distinct golden coat.

Incomplete Dominance vs. Codominance

It's crucial to distinguish incomplete dominance from codominance. In codominance, both alleles are fully expressed simultaneously, without blending. A classic example is the AB blood type in humans; both A and B alleles are expressed equally. In incomplete dominance, the alleles blend, creating an intermediate phenotype.

Punnett Squares and Incomplete Dominance

Predicting the offspring's phenotypes in incomplete dominance involves using Punnett squares, just like in complete dominance. However, the resulting phenotypes reflect the blend of alleles.

What is a Test Cross? How does it relate to incomplete dominance?

A test cross involves breeding an individual with a dominant phenotype (but unknown genotype) with a homozygous recessive individual. This helps determine the genotype of the dominant phenotype individual. In incomplete dominance, a test cross can easily reveal if an individual with an intermediate phenotype is heterozygous.

Conclusion: The Significance of Incomplete Dominance

Incomplete dominance highlights the complexities of inheritance beyond simple dominant and recessive relationships. Understanding incomplete dominance is essential for comprehending the diversity of genetic expression and its impact on observable traits across various species. It expands our understanding of the interplay between genes and the environment in shaping an organism's phenotype. Further research into incomplete dominance and other non-Mendelian inheritance patterns continues to refine our understanding of genetics and evolution.