Another important development enhancing the possibilities of genetic animal models of alcoholism was the development of transgenic animals in the late 1980s. These are animals that have been genetically modified so that the expression of a single candidate gene has been selectively inactivated or augmented compared with the parent strain. This approach allows researchers to study the influence of individual genes on risk for alcoholism (or many other diseases or behaviors). By now, more than 100 candidate genes have been studied for their contribution to alcohol’s effects, usually by comparing mice in which a single gene has been inactivated (i.e., knockout mice) with control mice in which the gene still is functional. As reviewed by Crabbe and colleagues (2006), most of the genes thus studied were found to influence some aspect of alcohol sensitivity. For example, of 84 different transgenic animals tested for effects on alcohol self-administration, one-quarter exhibited increased drinking, one-third exhibited decreased drinking, and 40 percent did not differ from control animals (Crabbe et al. 2006). This finding clearly demonstrates the multiplicity of genetic influences on alcohol responses.
