Which type of genetic allele expresses its characteristic phenotype only when identical?

The correct answer is recessive. A recessive allele is one that does not manifest its traits in the organism's phenotype unless two copies of that allele are present, meaning that the individual must be homozygous for the recessive trait for it to be expressed. In other words, both alleles inherited from the parents must be the same recessive allele for the characteristic to show up.

In contrast, a dominant allele will express its phenotype even when only one copy is present, meaning it can overshadow the effect of a recessive allele if both are present. Co-dominant alleles express both phenotypes simultaneously when present together, and incomplete dominance results in a blended phenotype of the two alleles rather than showing one trait or the other. This is why the understanding of recessive alleles is foundational in genetics; they require both alleles to be identical to be expressed in the phenotype.