Many genes are quite flexible in their effects. For example, the action of a gene quite often depends on the environment in which it finds itself. The distinction that has often been made between ΓÇÿnatureΓÇÖ (i.e., genes) and ΓÇÿnurtureΓÇÖ (i.e., the environment) is too simplistic to be useful. Genes can be compared to a recipe, but they are more than that. They are switched on and off all the time so that the right proteins can be made at the right time. Many genes actually interact with the environment all the time, in feedback loops, allowing a changing response to changing conditions. The development of embryos is particularly susceptible to subtle changes in the environment; whether or not the mother drinks or smokes during pregnancy can alter the expression of the childΓÇÖs genes forever.
Of course, the extent to which a geneΓÇÖs expression is flexible varies from gene to gene. Eyes do not change colour once they are developed. But body weight, or even intelligence, can change as the environment changes. Of course, there are limits. If the environment changes too much, an organism will probably die.
