Michigan Test for Teacher Certification (MTTC) Secondary Integrated Science Practice test

What genetic principle is demonstrated when neither gene masks the other and a new phenotype is formed?

1. Codominance

2. Incomplete dominance

3. Epistasis

4. Polygenic inheritance

The correct answer is: Codominance

The correct principle demonstrated when neither gene masks the other and a new phenotype is formed is known as incomplete dominance. In this genetic scenario, the alleles of two different genes blend together in the offspring, resulting in a phenotype that is a mixture of the parental traits rather than a display of one trait over the other. For example, if one parent plant has red flowers and another has white flowers, the offspring may exhibit pink flowers instead of either the solid color of the parents. This blending effect is characteristic of incomplete dominance, as neither of the original flower colors is fully expressed. In contrast, codominance involves both alleles being fully expressed in a phenotype, resulting in a distinct combination, but without blending. Epistasis refers to the interaction between genes where one gene can mask the expression of another gene, and polygenic inheritance involves multiple genes contributing to a single trait, leading to a wide range of phenotypes. Each of these concepts differs from how incomplete dominance operates, where the intermediate phenotype distinctly arises from the blending of alleles rather than a masking or independent expression.