A dominant allele is a variant of a gene that expresses its trait in the presence of another allele. When an individual possesses at least one dominant allele, the dominant trait is phenotypically expressed. This principle is foundational in Mendelian genetics, where dominant-recessive relationships can be observed. For example, if a dominant allele codes for a specific color in flowers, the presence of this allele will result in the flower displaying that color, regardless of the second allele's dominance. Understanding dominant alleles helps in predicting genetic outcomes.