Which of the following describes a dominant allele?

A dominant allele is characterized by its ability to express its trait whenever it is present in an organism's genotype. This means that even a single copy of a dominant allele is enough for its corresponding trait to manifest in the phenotype, which is the observable expression of traits. For example, in genetics, if an individual carries one dominant allele for a specific trait and one recessive allele, the dominant trait will be the one that is expressed.

This understanding is foundational in genetics, illustrating how dominant alleles can mask the presence of recessive alleles. In contrast, certain options suggest different characteristics of alleles that do not align with the definition of dominance. For instance, an allele that requires two copies to express its trait describes a recessive allele, while an allele with no effect on phenotype would signify that it is either silent or non-functional. Finally, the placement of an allele on a sex chromosome refers more to its inheritance pattern rather than its expression characteristics.