example of incomplete dominance

2 min read 20-03-2025

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 an intermediate blend of the two homozygous phenotypes. This differs from complete dominance, where one allele completely overshadows the other. Let's explore some compelling examples of incomplete dominance in the natural world.

Understanding Incomplete Dominance

Before diving into specific examples, let's solidify our understanding of incomplete dominance. In this type of inheritance:

Homozygous individuals: Show the typical expression of either allele. For example, a plant with two alleles for red flowers will have red flowers. A plant with two alleles for white flowers will have white flowers.
Heterozygous individuals: Display a blended phenotype. In our example, a plant with one allele for red and one for white flowers would have pink flowers. Neither red nor white completely dominates. The pink phenotype is a mixture of both.

This blending contrasts with complete dominance where the heterozygote displays the dominant allele's phenotype completely.

Striking Examples of Incomplete Dominance

Several examples illustrate incomplete dominance beautifully across diverse organisms:

1. Flower Color in Snapdragon Plants ( Antirrhinum majus)

Snapdragons are a classic example used to teach incomplete dominance. The cross between a homozygous red-flowered plant (RR) and a homozygous white-flowered plant (rr) produces offspring with pink flowers (Rr). This pink color is the intermediate blend of red and white, clearly demonstrating incomplete dominance. A cross between two pink snapdragons (Rr x Rr) will yield a phenotypic ratio of 1 red: 2 pink: 1 white.

2. Coat Color in Andalusian Chickens

Andalusian chickens offer another excellent example. Black (BB) and white (bb) chickens crossed result in blue Andalusian chickens (Bb). The blue color is a result of the incomplete dominance of black and white feathers. Similar to snapdragons, a cross between two blue Andalusian chickens will yield a 1:2:1 ratio of black, blue, and white offspring.

3. Palomino Horses

The beautiful palomino horse exemplifies incomplete dominance in mammals. Palominos possess a golden coat with a flaxen mane and tail. This unique coat color arises from incomplete dominance between a chestnut allele (CrCr) and a cremello allele (Crcr). A homozygous chestnut horse is a deep reddish-brown, while a homozygous cremello horse is a very pale cream color. The heterozygous (CrCrcr) palomino horse displays an intermediate color between the two extremes.

4. Human Hair Type

Although genetic factors influencing hair texture are complex, some aspects can be viewed through an incomplete dominance lens. A parent with straight hair (homozygous for straight hair) and a parent with curly hair (homozygous for curly hair) may have children with wavy hair. The wavy hair represents an intermediate phenotype between straight and curly, hinting at incomplete dominance. However, other genes also affect hair texture.

Beyond Simple Examples

It's important to remember that incomplete dominance is a simplification. Many traits are influenced by multiple genes (polygenic inheritance), creating complex interactions. The environment also often plays a significant role in phenotypic expression. While these examples highlight the core principle, the reality is often more nuanced.

Conclusion

Incomplete dominance showcases the fascinating complexity of inheritance. It challenges the simple dominant/recessive paradigm, highlighting how alleles can interact to produce a range of intermediate phenotypes. By understanding incomplete dominance, we gain a deeper appreciation of the intricate mechanisms governing heredity. The examples discussed, from the vibrant flowers of snapdragons to the unique coats of palomino horses, powerfully demonstrate this concept. They provide compelling evidence of how genes interact to create the diversity we see in the biological world.