Thus, linkage studies have identified some novel candidate genes for future investigation. These include genes, such as the cannabinoid receptor 1 (CNR1) gene, which are known for their biological relevance in cannabis use disorders that are discussed in detail below. However, after identifying genes under a linkage peak investigators need to determine how far from the linkage peak they wish to look. They need to keep in mind that each centiMorgan (the units in which genetic distance in linkage analysis is measured) roughly corresponds to a million base pairs in humans. This means that a linkage region of 20−30 cM may harbor over 300 genes of medium size (100kb or so) and the selection of one gene as a biologically plausible candidate over another can be difficult because of our limited knowledge of cannabis use disorders. For these reasons, and others (including statistical power, the complexities of conducting family-based studies) many investigators prefer to do gene association studies.
