One must underline that the environment acts not only additively to genetic background but also in combination with it. This is because the effect of an allelic variation of any gene depends on the environment in which the gene is expressed. The environment may be internal (e.g. gender) or external (e.g. diet, stress) to the individual, special to a particular time, or the accumulation of environmental experiences over the lifetime of the individual [40]. The environment can influence phenotype through the stimulation of gene expression, or via the interaction with gene products. For example, high salt intake, as an environmental factor, alters the expression of the renin gene, which ultimately leads to increased sodium excretion and, hence, prevention of the hypertensive effects of salt. Over a long period, the genetic component becomes dependent upon the environment because the environment can change the functional state of genes, and it is necessary to take into consideration a hierarchy of genotype, environment and phenotypes. The environment can influence phenotypes either indirectly through the stimulation of gene expression via mutations, or directly via interaction with gene products. Until recently, the effects of individual genes were considered to be independent from each other. This prediction is valid only for simple qualitative traits, in which the environment as a factor can be ignored. For quantitative traits, the environment is important and, therefore, genes become dependent variables.
© 2001 Alexander Binder