Genes and Environment:
There exists an intimate relationship between the gene, the genetic makeup of the individual (genotype), and the environment to which the individual is exposed. The ultimate phenotype is the sum total or net effect of all these factors. So, even if an individual possesses a certain genotype, there is no guarantee that the genotype will exhibit 100 percent penetrance and full expressivity.
The degree to which a certain gene expresses itself into a phenotype, in a population, or in an individual depends on the interacting influences of genotype and environment. The genotype contains genes and the action of many of them are interrelated. Thus, genes modify one another’s effect.