what is a incomplete dominance

Natashya

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15-03-2025

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Incomplete dominance, also known as partial dominance, is a type of inheritance where one allele doesn't completely mask the effects of another. This results in a heterozygous phenotype that's a blend of the two homozygous phenotypes. Unlike complete dominance, where one allele completely overshadows the other, incomplete dominance creates a third, intermediate phenotype. Let's delve deeper into this fascinating genetic concept.

Understanding Complete Dominance vs. Incomplete Dominance

Before exploring incomplete dominance, it's helpful to contrast it with complete dominance.

Complete Dominance: In complete dominance, one allele (the dominant allele) completely masks the expression of another allele (the recessive allele). For example, in pea plants, the allele for purple flowers (P) is dominant over the allele for white flowers (p). A plant with the genotype Pp will have purple flowers because the P allele completely masks the p allele.

Incomplete Dominance: In incomplete dominance, neither allele is completely dominant. The heterozygote displays a phenotype that is intermediate between the two homozygous phenotypes. This creates a third, distinct phenotype not seen in either parent. A classic example is flower color in snapdragons.

Examples of Incomplete Dominance

Several examples illustrate the concept of incomplete dominance across different species:

• Snapdragons: Red snapdragons (RR) crossed with white snapdragons (rr) produce pink snapdragons (Rr). The pink color is an intermediate phenotype resulting from the incomplete dominance of the red and white alleles. If two pink snapdragons (Rr) are crossed, you get a phenotypic ratio of 1 red: 2 pink: 1 white.

• Carnations: Similar to snapdragons, carnation flower color exhibits incomplete dominance. A cross between a red carnation and a white carnation will result in pink offspring.

• Palomino Horses: The coat color of Palomino horses is another example. A cross between a chestnut horse (homozygous for the chestnut allele) and a cremello horse (homozygous for the cremello allele) results in a Palomino horse with a distinctive golden coat and flaxen mane and tail. The Palomino phenotype is a blend of the chestnut and cremello phenotypes.

• Andalusian Chickens: The Andalusian fowl shows a similar pattern of incomplete dominance in feather color. Blue Andalusian chickens are heterozygous, arising from crosses between black and white chickens.

How Incomplete Dominance Works at the Molecular Level

The mechanism of incomplete dominance often involves the amount of gene product produced. The heterozygote produces only half the amount of functional protein compared to the homozygote expressing the dominant allele. This reduced protein level leads to the intermediate phenotype. For example, in snapdragons, the red allele might code for a pigment protein. A heterozygote produces half the amount of pigment protein, resulting in the pink color.

Distinguishing Incomplete Dominance from Other Inheritance Patterns

It's crucial to differentiate incomplete dominance from other genetic phenomena:

• Codominance: In codominance, both alleles are fully expressed in the heterozygote. There's no blending; rather, both traits are distinctly visible. A classic example is AB blood type in humans, where both A and B antigens are expressed.

• Multiple Alleles: Incomplete dominance can involve multiple alleles, but it's the interaction between the specific allele pairs that determines the incomplete dominance effect.

• Epistasis: Epistasis involves the interaction of different genes, affecting the expression of another gene. Incomplete dominance, on the other hand, focuses on the interaction of alleles within a single gene.

Conclusion

Incomplete dominance presents a compelling exception to the simple Mendelian inheritance patterns of complete dominance. Understanding incomplete dominance provides valuable insights into the complexities of gene expression and inheritance, highlighting the diverse ways genes interact to shape an organism's phenotype. By recognizing the intermediate phenotype characteristic of incomplete dominance, we can further refine our understanding of inheritance patterns across various species.