What is the effect of a single copy of an allele exhibiting complete dominance?

A single copy of an allele exhibiting complete dominance will completely mask the effect of a recessive allele. In this genetic scenario, when an organism possesses one dominant allele, the traits associated with the recessive allele are not expressed at all. This means that regardless of whether the organism has one or two copies of the dominant allele, the presence of that dominant allele ensures that the phenotypic outcome reflects the dominant trait. The recessive trait only manifests when an organism possesses two copies of the recessive allele.

The other options do not accurately describe the behavior of dominant alleles. For instance, a dominant allele does not enhance the effects of recessive alleles nor does it have no effect on them; instead, it effectively inhibits the expression of the recessive allele. Moreover, complete dominance does not lead to partial expression of both alleles, which is characteristic of codominance or incomplete dominance. Thus, the correct response highlights the exclusive influence of the dominant allele in overriding the recessive one.