Final answer:
A gene with two dominant alleles that are expressed at the same time is referred to as codominance, exemplified by the human blood type AB.
Step-by-step explanation:
When an organism inherits a gene with two dominant alleles that are expressed at the same time, the gene expression pattern is referred to as codominance. In codominance, both alleles are expressed equally in the phenotype of the heterozygote. This contrasts with complete dominance, where one allele completely masks the other, and incomplete dominance, where the phenotype is intermediate between the two homozygotes. An example of codominance is the human blood type AB, where both A and B alleles are expressed.