What is the name of the allele that is expressed only when two copies are present?

The allele that is expressed only when two copies are present is known as a recessive allele. This type of allele requires both copies—one from each parent—to manifest its trait in the phenotype of an organism. For example, if a trait is controlled by a recessive allele, an individual must inherit this allele from both parents in order for the trait to be expressed. In cases where an individual has only one copy of the recessive allele, the dominant allele (if present) will mask the effect of the recessive allele, preventing its expression.

In contrast, dominant alleles are those that can express a trait even when only one copy is present. Codominant alleles lead to both traits being expressed simultaneously in the phenotype. Incomplete dominance results in a blending of traits rather than one trait dominating over the other. The distinction of recessive alleles is fundamental to understanding inheritance patterns and how traits are passed from one generation to the next.