Reading your updated question, I think you are interested about epistasis. Let me know if I am mistaken.
The concept of epistasis might be confused with the one of pleyotropy, so I will define both terms.
Pleyotropy occurs when one gene influences multiple phenotypic traits. Note: the phenotype, to make it simply is what a living things look like (its cells, proteins, shape of the face, size of the leaves, etc…). It is often thought in duality with the genotype which is the total genetic information contained in an individual that codes for the phenotype (Be careful, the environment is also an important factor influencing the phenotype, if you make sports you will have big muscles!)
Epistasis is when the effect on the phenotype of one gene depends on the presence of several (or only one) other genes. Wikipedia gives this (simplistic) example:
Here, two bi-allelic genes are involved. One gene that determines the hair color. The two alleles influence the phenotype such: one allele gives red hair and the other allele gives blond hair. Now There's another bi-allelic gene coding for baldness. One allele causes the bearer to be hairy and the other one causes the bearer to be bald. Therefore, if an individual carries the allele "Bald" in the "Baldness" loci, what this individual carries on the other loci is not important because it will result into the same phenotype anyway, he is bald. Now if in the baldness loci, the allele is "Hairy", then the expression of the other gene "Hair color" matters and therefore if the allele is "blond", the bearer has blond hair and if the allele is "red" the bearer has red hair.
The "Baldness" gene is called a modifier. One can also say that the expression the "Hair color" gene depends on the genetic background of the individual.
Now cases of epistasis are not always that easy and if they influence the fitness, this kind of epistatic interaction might have implications for their evolution.