What is an inheritance pattern in which the alleles are neither dominant nor recessive called?

Codominance. Unlike the ones that Mendel studied in his pea plants, not all alleles are dominant and recessive. When both alleles are equally stong and both alleles are apparent to the eye, this is called codominance.

A popular illustration of codominance is chicken born from a hen with white feathers and rooster with black feathers. The result? A chicken that has both black feathers and white feathers. Another example is a human with type AB blood. This means that the A allele and the B allele are both represented equally, creating the AB blood type.