Which allele is classified as recessive in the context of blood type inheritance?

In the context of blood type inheritance, the O allele is classified as recessive. This means that an individual must inherit two O alleles (one from each parent) in order to express the O blood type. In contrast, the A and B alleles are both dominant, meaning that if one or both of these alleles are present in an individual’s genotype, they will express either the A or B blood type, respectively.

When examining blood types, the presence of the O allele does not contribute to the phenotype when paired with either A or B. Therefore, individuals with a blood type of AB express both A and B antigens and do not exhibit the characteristics of the O type at all. This dominant-recessive relationship in blood type inheritance shows why the O allele is uniquely classified as recessive.