Gene variants related to alcoholism risk that are present in a population at low frequency are difficult to detect because the number of people who need to be genotyped increases dramatically. For example, the ALDH2*2 allele that has such a strong effect is essentially absent in many areas of the world and therefore is not detected in studies of most populations. However, the effect of ALDH2*2 on risk for alcoholism is easy to detect even in relatively small studies of populations in which it is common, for example in China and Japan. Likewise, the ADH1B*2 allele, which is very common in East Asia and relatively common in the Middle East, is relatively uncommon (i.e., generally has allele frequencies of less than 4 percent) in most other places. Thus, it is easy to detect the effect of ADH1B*2 even in small studies of Asian populations but much more difficult in other populations (Li et al. 2011). However, a recent study in which the ADH1B*2 allele was genotyped in several thousand people of European ancestry showed a highly significant protective effect, comparable
