'Gene-environment interaction' is another potentially confusing term commonly used to describe the environmental sensitivity of organisms. In quantitative genetic analyses, when variation in an environmental factor has different effects on individuals of different genotype there is said to be, in a statistical sense, a 'gene-environment interaction'. But this term has been transferred into more informal discussions of environmental sensitivity in statements like ''the phenotype is a product of geneenvironment interaction during development.'' Not only might this be confused with the precise usage of quantitative genetics, but it can be taken to imply that genes somehow interact directly with the external environment during development to produce a phenotype. In fact, developmentally important factors - both genes (gene products) and environmental factors - influence development through their effects on the responsive phenotype that exists prior to the impact of these factors. Thus, genetic and environmental factors 'interact' only indirectly, via their effects on the developing phenotype.
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