Examples of heritable phenotypes for which twin data document shared genetic determinants include frontal lobe brain volume and cognitive abilities [15, 117]. Examples of heritable phenotypes for which co-occurance makes shared genetics highly likely, a priori, include substance dependence and bipolar disorder [148, 149]. A possible “transitive” genetics comes from the shared genetics of substance dependence and cognitive abilities/brain volume on the one hand, and the likely shared genetics of cognitive ability/brain volume and vulnerability to Alzheimer’s disease, on the other hand. Data from cognitive function and frontal brain volume genetics thus provide potential intermediate phenotypes to link the genetics of addiction with that of Alzheimer’s disease. Such links might or might not have been anticipated, based on equivocal evidence from current epidemiologic studies [150].
