Definition of Incomplete Dominance
Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not entirely dominant over the other allele. This results in a third phenotype that is a blend or an intermediate expression of the two parental phenotypes. Unlike complete dominance, where one allele completely masks the other, neither allele is fully expressed, leading to a mixed observable characteristic.
How It Differs from Complete Dominance
In complete dominance, an individual heterozygous for a trait will express the dominant phenotype, while the recessive allele remains hidden. With incomplete dominance, a heterozygote exhibits a phenotype that is distinct from, and often intermediate to, the phenotypes of the two homozygous parents. For example, if a red flower (homozygous dominant) crosses with a white flower (homozygous recessive), the offspring might be pink, demonstrating a blend rather than just red or white.
A Classic Example: Four O'Clock Flowers
A common example of incomplete dominance is observed in Mirabilis jalapa, or "four o'clock flowers." When a homozygous red-flowered plant (RR) is crossed with a homozygous white-flowered plant (WW), all the F1 generation offspring will have pink flowers (RW). If these pink F1 plants are self-pollinated or crossed with each other, the F2 generation will produce red, pink, and white flowers in a 1:2:1 phenotypic ratio, demonstrating the re-emergence of the parental traits and the intermediate phenotype.
Importance in Understanding Genetic Variation
Understanding incomplete dominance is crucial for a comprehensive view of genetic variation and how traits are passed down through generations. It illustrates that genetic inheritance is not always a simple "either/or" scenario but can involve a spectrum of expression. This pattern highlights the complex interactions between alleles and contributes to the diversity of phenotypes observed in nature, affecting traits ranging from flower color to human hair texture and cholesterol levels.