Missing heritability in AD GWAS is also attributable to the fact that AD’s liability is genetically heterogeneous (i.e., different individuals posses different combinations of susceptibility alleles within/across genes) and phenotypically heterogeneous (i.e., individuals might arrive at a diagnosis of AD with a combination of different characteristics/symptoms). This largely reflects the fact that people become addicted or remain addicted to alcohol for different reasons. Hence, the lack of power in alcohol GWAS AD can also be attributed to the use of phenotypes that fail to capture the biological underpinnings of AD, which would ultimately led to the classification of groups/types of alcoholics that may be more genetically homogeneous. This is primarily obvious in summary phenotypes, such as AD, which combine physiological characteristics of the disorder with psychosocial aspects. This, in turn, results in different combinations of individuals with different problems. Consequently, every GWAS of AD has had to average the score across individuals with different aspects of an underlying inability to regulate their alcohol consumption. For example, a recent GWAS by Kendler et al. (2011) indicated that while symptoms of AD
