Question

which statement describes complete dominance when different alleles are present?
a. both traits are shown at the same time.
b. the phenotype is an intermediate combination.
c. only one allele determines the phenotype.
d. the phenotype is controlled by multiple alleles.

Explanation:

Complete dominance is a situation in genetics where in a heterozygous genotype (having two different alleles), the dominant allele completely masks the effect of the recessive allele. So, only the trait of the dominant allele is visible in the phenotype.

• Option A describes codominance (e.g., roan cattle where both red and white hair traits are shown).
• Option B describes incomplete dominance (e.g., snapdragons where a red - flowered and a white - flowered parent produce a pink - flowered offspring).
• Option C is correct for complete dominance as the dominant allele's phenotype is the only one expressed when different alleles (heterozygous) are present.
• Option D describes a situation like human blood types (ABO system) which is an example of multiple - allele inheritance, not complete dominance.

Answer:

C. Only one allele determines the phenotype.