In genetics, what represents two alleles that are neither dominant nor recessive?

The concept of two alleles being neither dominant nor recessive is best represented by incomplete dominance. In this genetic scenario, when an organism inherits two different alleles for a trait, the resulting phenotype is a blend of both alleles rather than exhibiting the characteristics of one allele over the other.

For instance, consider a situation where a plant has one allele for red flowers and another for white flowers. Instead of displaying either red or white, the flowers may appear pink, which is a clear indication of incomplete dominance. This blending effect occurs because neither allele fully masks the expression of the other, resulting in a phenotype that is intermediate between the two alleles.

Other terms in the choices refer to different genetic concepts. Co-dominance occurs when both alleles in a heterozygote are equally expressed, as seen in blood type inheritance where both A and B alleles can be present. Complex inheritance could refer to multifactorial traits influenced by multiple genes and environmental factors. Polygenic inheritance involves multiple genes contributing to a single trait, such as height or skin color, but does not imply that the alleles blend in their expression. Understanding these distinctions clarifies why incomplete dominance is the correct identification of the genetic interaction described.