Association studies compare the prevalence of a genetic marker (such as bloodtype) in groups of persons with or without bipolar disorder.  Using association studies allows for relative robustness to genetic heterogeneity and the ability to detect for much smaller effect sizes than are detectable in linkage studies.
    Millon, Blaney, & Davis (1999) suggest that the most commonly studied genetic traits have been ABO blood types, human leukocyte antigens, and the enzyme monoamine oxidase, but the studies have not found consistent results . Craddock, Dave, & Greening (2001) state that most candidate gene studies have focussed on the major neurotransmitter systems that are influenced by medication.  Rarely there may be families where one single gene may be responsible for determining susceptibility, but usually there is an interaction of multiple genes that cause bipolar disorder.  So far no specific gene has been confirmed as a susceptibility or modifying locus for bipolar disorder, and researchers have found modest effect sizes for susceptibility genes with association studies. (Craddock, Dave, & Greening, 2001)