Which of the following statements best captures Mendel's principle of dominance?

• A. A single gene determines the trait in all cases
• B. Some alleles are dominant and others are recessive
• C. Alleles are always inherited together
• D. Traits are determined solely by the environment

Answer: (B)

Dominance describes how alleles at a single gene interact to determine what phenotype appears in a organism when two different alleles are present. In a heterozygous individual, the dominant allele's trait is expressed while the recessive allele is masked. For example, tall (dominant) over short (recessive) in pea height means TT and Tt plants look tall, while tt plants are short. This shows why some alleles are dominant and others recessive.

The other statements don’t fit Mendel’s principle: a trait isn’t always determined by a single gene in every case, alleles aren’t always inherited together, and traits aren’t determined solely by the environment in Mendelian terms.