Genetics: Gene-Environment Interaction

In addition to the types of interaction cited above, there are other ways in which the effects of genes and environments are intertwined: environments can influence gene expression, and genes may affect the array of environments to which an organism is exposed (or exposes itself). These processes can lead to correlation between genotype and environment.

The influence of environment on the expression of genes was classically demonstrated in the operon model of Jacob and Monod, a landmark in the history of molecular genetics. Changes in the nutrient composition of the media of E. coli resulted in the "turning on" of the organism's gene that produces the appropriate enzyme for metabolism of lactose. Subsequent research on gene regulation has revealed that the particular genes of an organism that are being expressed may differ from time to time, both developmentally and in response to environmental factors. There has been, for example, a burgeoning of information about the effects of various stressors on gene expression. Described as "heat shock genes" because of the early experimental situations involving brief administration of a high-temperature environment to bacteria and Drosophila, this literature now includes examples of various environmental stresses both in vitro and in vivo, and in several species. As a general summary, these stressors have the effect of inhibiting the typical ongoing protein production of the cells, and promoting the translation of genes that produce a class of proteins that have a protective function in the cells. The rich detail of this research area complements the generalization that has emerged from quantitative genetic analyses in a variety of species, plant as well as animal, that the heritability of quantitatively distributed traits—the portion of the phenotypic variance attributable to genetic differences among the organisms—is often increased under conditions of environmental stress (Hoffman and Parsons).

It has long been appreciated that different loci are expressed in different tissues, and that different loci may be active at different developmental periods. The observations of Rogina and Helfand, who have described a typical life-span pattern of expression of a particular locus in the antennae of Drosophila, are particularly pertinent to the conceptualization of genetic and environmental influences on aging. Beginning at low levels, mRNA from this locus rises to a peak at midlife, with a subsequent decline to the initial low levels. In different temperature environments that influence Drosophila life span, the rising and falling of the mRNA level is altered, but the form of the function relative to the total life span under the particular environmental circumstances is remarkably preserved. This result can be interpreted as identifying an "intrinsic" pattern of gene expression over the life span, the temporal parameters of which are strongly influenced by environmental temperature. Another age-related illustration of environments affecting gene expression is that of Lee and colleagues, who used a gene array of the gastrocnemius muscle of mice to describe differences in the gene expression profile at five months and thirty months of age. Under conditions of caloric restriction, well established as a life-extending environmental intervention, most of the described gene expression changes were prevented or delayed.

There is also a growing literature concerning the role of genetics in determining the environment to which an organism is exposed. The field of microhabitat selection has provided plentiful illustration of organisms seeking environmental circumstances most suited to some aspect of their gene-influenced physiology. A case in point is that of selection of environments with or without the presence of alcohol by Drosophila larvae with different genotypes affecting alcohol dehydrogenase activity (Cavener). The scope for selecting from the array of environmental niches (or making them) is particularly pronounced in human beings (see Bergeman et al.).