Which type of allele only manifests its effect on phenotype in the homozygous state?

A recessive allele is one that only expresses its phenotype when paired with another recessive allele, which occurs in the homozygous state. In other words, an individual must have two copies of the recessive allele (one from each parent) for the trait associated with that allele to be visible in their phenotype. For instance, if a trait such as flower color is governed by a recessive allele, the flowers will only exhibit that particular color if the individual has both alleles that code for it; if the individual has only one recessive allele and one dominant allele, the dominant trait will prevail.

In contrast, a dominant allele will manifest its effect in either the homozygous or heterozygous state, meaning it can express the associated trait even if there is only one copy present. Codominant alleles express both traits simultaneously in the phenotype when present together. Epistatic alleles may affect the expression of other genes, but their effect is not limited to the homozygous state. Thus, the defining characteristic of a recessive allele is its requirement for two copies to express the phenotype.