Learning Outcomes
i. Define incomplete dominance and differentiate it from complete dominance and co-dominance.
ii. Describe the phenotype of heterozygotes in incomplete dominance using the example of the Japanese 4 O'Clock plant.
iii. Understand the genetic ratios resulting from crosses involving incomplete dominance.
i. Incomplete Dominance Defined
Incomplete dominance is a type of genetic inheritance where the phenotype of the heterozygote is intermediate between the phenotypes of the two homozygotes. This means that in the heterozygous condition, neither allele is completely dominant over the other, resulting in a blend of the two traits.
ii. Japanese 4 O'Clock Plant
The Japanese 4 O'Clock plant (Mirabilis jalapa) serves as an excellent example to explain incomplete dominance. In these plants, the flower color trait has two alleles: one for red flowers (R) and one for white flowers (W). Unlike complete dominance where one allele is dominant over the other, in incomplete dominance, the heterozygous plants (RW) have pink flowers—a mix of red and white.
When two pink-flowered plants (RW) are crossed, the offspring have a 1:2:1 genotypic ratio (1 RR: 2 RW: 1 WW) and a corresponding phenotypic ratio where 25% have red flowers, 50% have pink flowers, and 25% have white flowers. This 1:2:1 ratio is typical for incomplete dominance scenarios.
iii. Genetic Ratios in Incomplete Dominance
Incomplete dominance leads to a characteristic 1:2:1 phenotypic ratio in the F2 generation when two heterozygotes are crossed. This is because the alleles mix rather than one being completely dominant over the other. The intermediate phenotype is not a result of both alleles being expressed, as in co-dominance, but rather a blending of the two.
In summary, incomplete dominance is a form of inheritance where the heterozygote's phenotype is an intermediate of the two homozygous phenotypes, neither allele being completely dominant. This concept is vital for understanding the diversity of inheritance patterns beyond the classical dominant-recessive framework and provides insight into how genetic traits can vary in a population.
