Practice question

Duplicate dominant interaction implies functional equivalence between two loci where dominant allele at either locus complements loss at other locus because both control same biochemical step or parallel pathways. Presence of dominant genotype at first locus A- or second locus B- yields identical phenotype such as capsule dehiscence, awn formation, or pigment production. Only double recessive aabb lacks functional product, producing contrasting phenotype. Therefore dominant alleles interchangeable, explaining 15:1 ratio and protection against single-gene mutation, differing from complementary interaction requiring both dominants simultaneously for novel phenotype expression.