What characterizes a recessive allele?

A recessive allele is characterized by its expression only when an individual has two copies of that allele, which is known as being homozygous. In a heterozygous state, where an individual has one dominant allele and one recessive allele, the dominant allele masks the effect of the recessive allele, preventing it from being expressed in the phenotype. This is why the recessive trait is not observed unless the individual is homozygous for that trait. The homozygous condition ensures that there are no competing dominant alleles, allowing the recessive phenotype to manifest. Thus, a characteristic feature of a recessive allele is that it requires both copies to be the same and recessive for the associated trait to be visible in the organism.