Step 1: In a standard dihybrid cross, independent assortment normally gives a 9:3:3:1 ratio, but when the two genes involved affect the same trait in a redundant way, this ratio gets modified.
Step 2: If having a dominant allele at either one of the two loci (or both) produces the same visible phenotype, then only individuals that are homozygous recessive at both loci (1/16 of offspring) show the different, recessive phenotype, combining the 9, 3 and 3 classes into 15 parts showing one phenotype and 1 part showing the other.
Step 3: Since the two genes duplicate each other's phenotypic contribution rather than one masking the other (as in epistasis) or suppressing it (as in inhibitory action), this specific 15:1 outcome is named duplicate gene interaction.
The correct answer is Duplicate factor.