available (Mardones and Segovia-Riquelme 1983). Over the course of several days, animals develop a stable and characteristic intake of ethanol, which can be indexed as either the relative preference for ethanol (the preference ratio, or PR, equals the percentage of total daily fluid consumed from the ethanol bottle) or as the dose of ethanol ingested, usually expressed as g ethanol/kg body weight per 24-h or during a limited time period per day. The latter index is preferable because it is relatively independent of the percentage of alcohol offered. The success of this first selection proved, ipso facto, that genes contribute to alcohol preference drinking, and the project spawned a large number of other selected lines of rats and mice bred for the same phenotypic difference in preference drinking.
