Gene for X

… what does it mean to say there is a genetic component to the difference, in common parlance that there is a gene (or genes) ‘for’ homosexuality? It is a fundamental truism, of logic more than of genetics, that the phenotypic effect of a gene is a concept that has meaning only if the context of environmental influences is specified, environment being understood to include all the other genes in the genome. A gene ‘for’ A in environment X may well turn out to be a gene for B in environment Y. It is simply meaningless to speak of an absolute, context-free, phenotypic effect of a given gene. —p38, The extended phenotype