What distinguishes a dominant allele from a recessive allele?

A dominant allele is characterized by its ability to express its trait in the phenotype even when only one copy is present in the genotype, which means it can mask the effect of a recessive allele. This property signifies that if an individual carries one dominant allele, the trait associated with that dominant gene will be observed, regardless of whether the other allele is dominant or recessive. This is fundamentally important in understanding Mendelian genetics, where inheritance patterns can be predicted based on the presence of dominant and recessive alleles.

The other options misunderstand the relationship between dominant and recessive alleles. A recessive allele can be expressed, but only under specific conditions—specifically, when an individual has two copies of that recessive allele. The assertion that a dominant allele is always more beneficial is not universally true, as the effects of an allele can vary based on environmental factors and contexts; alleles can have neutral, beneficial, or harmful effects. Therefore, the key distinction is the ability of a dominant allele to express its trait independently of the other allele present, which makes option A the most accurate choice.