Which statement describes the principle of dominance?

Which statement describes the principle of dominance?

• A. All alleles are equally expressed in all genotypes
• B. Some alleles are dominant and others are recessive
• C. Dominant alleles are always expressed in heterozygotes
• D. Dominance refers to chromosome number

Answer: (B)

Dominance describes how alleles interact to shape an organism’s phenotype. In a heterozygous pairing, the allele that is dominant can mask the effect of the other allele, so the trait associated with the dominant allele appears even though a recessive allele is also present. This is why a single copy of a dominant allele can determine the visible trait, while two copies of the recessive allele are needed to express the recessive trait. For example, in the classic tall vs short trait, tall is dominant to short. A plant with one tall and one short allele will be tall because the dominant allele’s phenotype is expressed. There are, of course, nuances like incomplete dominance or codominance in some gene interactions, but the general idea is that some alleles are dominant and others recessive. The other statements don’t fit because dominance is about how alleles at a gene influence phenotype, not about all alleles being equally expressed in every genotype; and dominant alleles aren’t described as always being expressed in heterozygotes in every possible genetic context (there are exceptions). Also, dominance has nothing to do with chromosome number.

Dominance describes how alleles interact to shape an organism’s phenotype. In a heterozygous pairing, the allele that is dominant can mask the effect of the other allele, so the trait associated with the dominant allele appears even though a recessive allele is also present. This is why a single copy of a dominant allele can determine the visible trait, while two copies of the recessive allele are needed to express the recessive trait.

For example, in the classic tall vs short trait, tall is dominant to short. A plant with one tall and one short allele will be tall because the dominant allele’s phenotype is expressed. There are, of course, nuances like incomplete dominance or codominance in some gene interactions, but the general idea is that some alleles are dominant and others recessive.

The other statements don’t fit because dominance is about how alleles at a gene influence phenotype, not about all alleles being equally expressed in every genotype; and dominant alleles aren’t described as always being expressed in heterozygotes in every possible genetic context (there are exceptions). Also, dominance has nothing to do with chromosome number.