Chunk #18 — RESULTS — Genetically informative research approaches and their limitations — The relationship between genetics of use and genetics of abuse/dependence?
Relative to alcohol, Fowler and colleagues reported significantly greater covariance between use and misuse of cigarettes and cannabis 41. Perhaps the most convincing support for genetic overlap across stages of substance use arises from the study of cannabis and other illicit drugs, where results from adult samples show evidence for substantial genetic overlap (50-80%).45,46 Gillespie et al46 reported that cannabis availability accounted for 92-96% of the shared environmental (and only a modest 2-3% of the genetic) influences on cannabis initiation and abuse. The study, thus, presented two key conclusions – (a) that cannabis availability is heritable (18%), which is presumably due to individual predispositions to environmental exposure (or gene-environment correlation) and (b) that drug availability may represent a majority of shared environmental variance (C) in twin studies of substance use.
