Which of the following describes the phenotype of a heterozygote in complete dominance?

• A. The phenotype is a mix of both alleles
• B. The phenotype can only show the dominant trait
• C. The phenotype shows no expression of the dominant allele
• D. The phenotype displays characteristics of both alleles

Answer: (B)

In complete dominance, a heterozygote carries two different alleles for a particular gene, one dominant and one recessive. The defining feature of this genetic relationship is that the dominant allele completely masks the effect of the recessive allele. As a result, the phenotype—the observable traits—of the heterozygote will be identical to that of the homozygous dominant individual, which expresses the dominant trait.

This means that when contrasting traits are present, such as tall (dominant) and short (recessive) in a plant, a plant with one allele for tallness and one for shortness will still exhibit only the tall phenotype. Hence, the heterozygote displays only the dominant trait and does not reveal any characteristic of the recessive trait in its outward appearance.

This understanding is pivotal in genetics, particularly when predicting offspring traits in breeding experiments, as it emphasizes the role of dominant alleles in determining phenotypic outcomes in cases of complete dominance.