In a dihybrid cross of two heterozygous individuals you epect a 9: 3: 3: 1 phenotypic ratio in the offspring, but observe a ratio of 9: 7. what is the most likely explanation?

Epistatic interaction of the two genes

Explanation:

In epistasis, the interaction between genes contradics one another, in essence, one gene masks the  interference with the expression of another gene.  As such any time two different genes contribute and have effects to a single phenotype and their effects are not merely additive, those genes are said to be epistatic.