The law of incomplete dominance describes a form of inheritance where one allele for a particular trait is not expressed completely over its paired allele, leading to a heterozygous phenotype that is an intermediate blend of the two homozygous parents. Unlike complete dominance, where the dominant allele fully masks the recessive one, incomplete dominance results in a third, distinct phenotype.
Understanding Incomplete Dominance
In simple terms, incomplete dominance means that neither allele is truly dominant or recessive. Instead, when both different alleles are present in a heterozygous individual, they contribute to a mixed or blended observable characteristic.
- Blending of Traits: The most defining characteristic is the blending of traits. For instance, if red and white are the parental colors, the offspring might exhibit pink.
- Intermediate Phenotype: The heterozygous individual displays a phenotype that falls somewhere between the two homozygous phenotypes.
- Distinct Ratios: In a monohybrid cross involving incomplete dominance, the genotypic ratio (1:2:1) is the same as the phenotypic ratio (1:2:1). This is because each genotype (homozygous dominant, heterozygous, homozygous recessive) corresponds to a unique phenotype.
Incomplete Dominance vs. Complete Dominance
To fully grasp incomplete dominance, it's helpful to compare it with complete dominance, the more commonly taught inheritance pattern.
| Feature | Complete Dominance | Incomplete Dominance |
|---|---|---|
| Heterozygous Phenotype | Shows the dominant trait | Shows an intermediate/blended trait |
| Allele Expression | Dominant allele fully masks the recessive allele | Neither allele completely masks the other; both contribute |
| Phenotypic Ratio (F2) | 3:1 (dominant:recessive) | 1:2:1 (homozygous dominant:heterozygous:homozygous recessive) |
| Genotypic Ratio (F2) | 1:2:1 (homozygous dominant:heterozygous:homozygous recessive) | 1:2:1 (homozygous dominant:heterozygous:homozygous recessive) |
| Example | Tall (TT) x Dwarf (tt) -> Tall (Tt) in peas | Red (RR) x White (WW) -> Pink (RW) in snapdragons |
Examples in Nature
Incomplete dominance is observed in various organisms, providing compelling examples of its mechanism.
- Snapdragons (Antirrhinum majus): A classic example involves flower color.
- Homozygous dominant plants (RR) produce red flowers.
- Homozygous recessive plants (WW) produce white flowers.
- Heterozygous plants (RW) produce pink flowers, an intermediate blend of red and white.
- A cross between two pink (RW) snapdragons will yield offspring in a 1 Red: 2 Pink: 1 White ratio.
- Four O'Clock Flowers (Mirabilis jalapa): Similar to snapdragons, these flowers also demonstrate incomplete dominance in their flower color, with red and white parents producing pink offspring.
- Andalusian Fowl: Feather color in this chicken breed shows incomplete dominance.
- Black (BB) parents crossed with White (WW) parents produce offspring with blue feathers (BW), a diluted, intermediate color.
Genetic Basis
From a molecular perspective, incomplete dominance often arises when the dominant allele produces a protein or enzyme, but not enough of it to achieve the full phenotypic effect. The heterozygous individual, having only one copy of this functional allele, produces half the amount of the protein compared to a homozygous dominant individual, resulting in an intermediate phenotype. The recessive allele, on the other hand, might produce a non-functional protein or no protein at all.
Understanding incomplete dominance enriches our comprehension of genetic diversity and how traits are passed down through generations, showcasing that inheritance patterns can be more nuanced than simple dominant-recessive relationships. For further reading, explore resources on Mendelian inheritance patterns.
