A species of birds from South America have feathers that are either blue, black, or both blue and black. There are only two alleles for the gene that codes for feather's color, A and U. Allele A controls black color, and the allele U controls blue color. The birds with blue and black feathers fully express the alleles A and U. Most likely, A and U are ______________ alleles.
Dominant
Recessive
Codominant
Incomplete dominant