In an attempt to maximize the power of the GAIN/IMAGE family-based GWAS, Lasky-Su et al. (2008b) published a study focusing on quantitative ADHD phenotypes using the Family-based Association Test (FBAT) suite of programs. Based on the 18 DSM-IV symptoms of ADHD the authors constructed 3 quantitative phenotypes using (1) the number of hyperactive–impulsive symptoms, (2) the number of inattentive symptoms, and (3) the total number of symptoms. These phenotypes were each tested under three different genetic models (additive, dominant, recessive). Furthermore, three additional quantitative traits were constructed based on the Conners ADHD Rating Scales and the PACS using FBAT-PC methodology (Lange et al. 2004c). This approach constructs a slightly different phenotype for every SNP in order to maximize the heritability at this SNP. Association testing was subsequently limited to the set of ten SNPs per genotype with the highest power to detect association derived from a screening algorithm in the program PBAT (Lange et al. 2004b; van Steen and Lange 2005). In total, this study performed 18 GWAS, increasing the multiple testing burden; correction for multiple testing was limited to the single GWAS.
