The studies discussed above established that there is a substantial genetic basis underlying the predisposition to develop drug addiction, but could not establish the genetic architecture underlying those effects, or the specific genes involved. Other genetic approaches have been used in attempts to identify specific genes involved in addiction, including both linkage and association methods. Linkage involves comparison of genomic markers in related individuals, while association involves comparisons of unrelated individuals. Initial work of this type focussed on specific genes or specific genomic regions, as opposed to genome wide approaches discussed later in this review.
