What inheritance pattern occurs when both alleles are fully expressed in a heterozygote?

The inheritance pattern described occurs when both alleles are fully expressed in a heterozygote, which is known as codominance. In codominance, neither allele is recessive, and both contribute to the phenotype in a distinguishable way. This can be observed in certain blood types, for example, where individuals with one allele for type A blood and another for type B blood express both traits simultaneously, resulting in type AB blood. The presence of both alleles leads to a unique phenotype that shows characteristics from both contributing alleles, rather than blending or one masking the other. This clear co-expression of traits characterizes codominance and distinguishes it from other inheritance patterns.