In summary, the COGA study provided evidence that certain regions of chromosomes 1 and 7 were linked with alcohol dependence in both the initial and replicate samples. Other DNA regions, however, may have contained susceptibility genes that were present in only one of the samples (i.e., a region on chromosome 2 in the initial sample and a region on chromosome 3 in the replication sample). It is still unclear why these differences in susceptibility genes exist, particularly because there are no prominent differences between the initial sample and the replication sample. One can speculate that many genes of modest effect influence the development of alcohol dependence. Because such genes will be difficult to detect consistently, it is possible that one gene may be identified in the initial sample and a different gene may be identified in the replication sample.
