Mendel's principle of dominance states:

• A. All alleles are dominant
• B. Some alleles are dominant and others are recessive
• C. Alleles never influence phenotype
• D. The environment determines which alleles are expressed

Answer: (B)

Dominance describes how one allele can mask the effect of another in determining phenotype. In Mendel’s view, for a given gene, there are two alleles. If one allele is dominant over the other, the trait associated with the dominant allele appears in the organism even when the other allele is present. So a heterozygote exhibits the dominant trait, while only a homozygous recessive individual shows the recessive trait. This predictable pattern lets you explain why some traits skip appearing in a generation or reappear in the next: the recessive form is hidden unless you have two copies of it. For example, if purple flower color is dominant over white, a plant with one purple and one white allele looks purple, and only plants with two white alleles end up white. The other statements don’t fit Mendel’s observations: not all alleles are dominant, alleles do influence phenotype, and while the environment can affect expression in some cases, it doesn’t define which allele is dominant.