What genetic phenomenon occurs when one gene is not completely dominant over another gene?

When one gene is not completely dominant over another gene, the phenomenon that occurs is known as incomplete dominance. In incomplete dominance, the offspring express a phenotype that is a blend or intermediate of the two parental traits. This can be seen in certain flower colors, such as when a red flower (RR) and a white flower (rr) produce pink flowers (Rr). The pink phenotype demonstrates that neither the red nor the white allele is completely dominant; instead, their effects blend to create a new expression.

In contrast, codominance refers to a situation where both alleles in a heterozygous individual contribute to the phenotype distinctly and simultaneously, such as in the case of blood types where both A and B alleles are expressed.

Lethal alleles can result in the death of an organism when present in a certain genotype, but they do not relate to dominance relationships in the same context.

Polygenic characters involve multiple genes contributing to a single trait, leading to a range of phenotypes, such as height in humans, but this is also distinct from the dominance relationship described in incomplete dominance.

Thus, the correct choice highlights the blending nature of phenotypes that occurs when there is no complete dominance of one allele over another.