Which type of allele is expressed in an organism's phenotype, masking the effect of the recessive allele when present?

The type of allele that is expressed in an organism's phenotype and can mask the effect of a recessive allele when present is the dominant allele. In genetics, alleles can interact in various ways based on their dominance relationships.

A dominant allele expresses its trait even when paired with a recessive allele. For instance, if an organism has one dominant allele (represented by a capital letter) and one recessive allele (represented by a lowercase letter), the dominant trait will be the one observed in the organism's phenotype. This occurs because the dominant allele produces the necessary proteins or traits that lead to visible characteristics, effectively overshadowing the influence of the recessive allele.

Other types of alleles, such as recessive alleles, require two copies (one from each parent) to be expressed in the phenotype, meaning they do not show their effect when a dominant allele is present. Codominant alleles can also be present, but they express both traits equally rather than masking one another. Homozygous refers to having two identical alleles for a given gene, which does not specifically concern whether those alleles are dominant or recessive. Thus, dominant alleles are specifically characterized by their ability to dictate the phenotype when present alongside a recessive allele