Final answer:
When two alleles are dominant and expressed equally in a heterozygote, the inheritance pattern is known as codominance, which allows both alleles to be distinctly expressed in the phenotype.
Step-by-step explanation:
When two alleles for a gene are dominant and expressed equally in heterozygotes, the trait displays a pattern of inheritance known as codominance. In codominance, both alleles are expressed simultaneously in the phenotype without blending, each allele maintaining its own effects. This differs from incomplete dominance where the phenotype is intermediate between the two. An example of codominance is the ABO blood group system, where alleles for both A and B blood types are expressed equally, resulting in the AB blood type. In contrast, traditional dominance follows Mendel's theory, where one dominant allele would mask the presence of a recessive allele in a heterozygote.