Final answer:
Phenotype ratios expressed in fractions, such as 9:3:3:1, indicate that multiple gene pairs contribute to an observed phenotype, with two-gene interactions typically producing a total of 16 different phenotypic outcomes.
Step-by-step explanation:
When phenotype ratios are expressed in fractions, this means that multiple gene pairs are controlling the observed phenotype. For instance, the 9:3:3:1 phenotypic ratio, which totals 16, is characteristic of a two-gene interaction in Mendelian inheritance, such as that seen in a dihybrid cross. This ratio indicates that each trait is the product of two genes interacting with each other but assorting independently. An example of this is the interaction between two genes where one acts as an epistatic gene. In mice, for example, the gene responsible for pigment production (C) can mask the expression of another gene (A) related to coat color. If the cc allele is present, it results in an albino phenotype regardless of the allele present at the A locus.