Which statement best describes a dominant allele?

Which statement best describes a dominant allele?

• A. It shows the trait when present in the genotype.
• B. It is always present in the population.
• C. It is found only in homozygotes.
• D. It never affects phenotype.

Answer: (A)

A dominant allele is one that determines the phenotype whenever at least one copy is present. This means a heterozygote with one dominant and one recessive allele will express the trait associated with the dominant allele, because the dominant version masks the recessive one. The idea isn’t about how common the allele is in the population or about needing two copies for expression. An allele can be dominant and still occur in a population at low frequency, and a dominant trait can be seen in heterozygotes, not only in homozygotes. Conversely, an allele that never affects phenotype would not be described as dominant. For example, if the tall-trait allele is dominant, someone with one tall allele and one short allele will be tall.

A dominant allele is one that determines the phenotype whenever at least one copy is present. This means a heterozygote with one dominant and one recessive allele will express the trait associated with the dominant allele, because the dominant version masks the recessive one. The idea isn’t about how common the allele is in the population or about needing two copies for expression. An allele can be dominant and still occur in a population at low frequency, and a dominant trait can be seen in heterozygotes, not only in homozygotes. Conversely, an allele that never affects phenotype would not be described as dominant. For example, if the tall-trait allele is dominant, someone with one tall allele and one short allele will be tall.