what is incomplete dominance in genetics

Daniel Parker

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18-03-2025

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Incomplete dominance, also known as partial dominance, is a type of inheritance where one allele for a specific trait does not completely dominate the other allele. This results in a third phenotype, which is a blend of the parental phenotypes. Unlike complete dominance, where the heterozygote displays the dominant phenotype, incomplete dominance shows a unique intermediate phenotype. This means the offspring exhibits a trait that's a mixture of the two parents' traits.

Understanding Alleles and Phenotypes

Before diving into incomplete dominance, let's review some fundamental genetic concepts.

• Alleles: These are different versions of a gene. For example, a gene for flower color might have one allele for red flowers (R) and another allele for white flowers (W).

• Genotype: This refers to the genetic makeup of an organism. It represents the combination of alleles an individual possesses (e.g., RR, RW, WW).

• Phenotype: This is the observable characteristic or trait of an organism (e.g., red flowers, pink flowers, white flowers).

In complete dominance, one allele (the dominant allele) masks the expression of the other allele (the recessive allele). In the flower color example, if R is dominant, RR and RW plants would both have red flowers, while only WW plants would have white flowers.

How Incomplete Dominance Works

In incomplete dominance, neither allele is completely dominant. When a heterozygote (having two different alleles) is formed, the resulting phenotype is a blend of the two homozygous phenotypes. Let's illustrate with the flower color example again:

• RR: Red flowers
• RW: Pink flowers (a blend of red and white)
• WW: White flowers

The pink flowers in the RW genotype demonstrate incomplete dominance. The red allele (R) doesn't fully suppress the white allele (W), leading to a mixed phenotype. This blending is characteristic of incomplete dominance.

Examples of Incomplete Dominance

Several examples of incomplete dominance exist in nature:

• Snapdragon Flowers: As mentioned above, snapdragons exhibit incomplete dominance in flower color. A cross between a red snapdragon (RR) and a white snapdragon (WW) produces pink snapdragons (RW).

• Four O'Clock Flowers: Similar to snapdragons, four o'clock flowers show incomplete dominance in flower color. Different combinations of red and white alleles result in red, pink, or white flowers.

• Human Hair: Some aspects of human hair texture might exhibit incomplete dominance. A person inheriting alleles for curly hair and straight hair might have wavy hair. However, this is complex and often influenced by multiple genes.

Distinguishing Incomplete Dominance from Other Inheritance Patterns

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

• Complete Dominance: One allele completely masks the other. The heterozygote displays the dominant phenotype.

• Codominance: Both alleles are fully expressed in the heterozygote. There's no blending; both traits are visible. An example is AB blood type, where both A and B antigens are present.

• Multiple Alleles: More than two alleles exist for a particular gene (e.g., human ABO blood group system).

Punnett Squares and Incomplete Dominance

Punnett squares are useful tools to predict the genotypes and phenotypes of offspring in incomplete dominance. Let's use the snapdragon example:

This Punnett square shows that crossing two pink snapdragons (RW) yields a 1:2:1 phenotypic ratio (1 red: 2 pink: 1 white).

Conclusion

Incomplete dominance is a fascinating inheritance pattern where neither allele is completely dominant over the other. Understanding this concept helps us appreciate the diversity of gene expression and the complexity of heredity. The blending of phenotypes in incomplete dominance stands in contrast to the complete dominance seen in many other traits. While relatively common, it’s crucial to note that the phenotypic expression is always impacted by environmental factors and can be influenced by other genes.