Give examples that could prove that the manifestation of a trait (phenotype) is the result of the interaction

Give examples that could prove that the manifestation of a trait (phenotype) is the result of the interaction of genes in the specific conditions of the organism’s existence.

In genes, information about proteins is encoded. Bepki form qualitative signs. “One gene, one protein.” If a gene mutates, then both the protein and the qualitative trait change after it. However, the expression “a gene determines the development of a trait” is conditional, since the action of a gene depends on other genes, on the genotypic environment.