For the current analyses, both alcohol phenotypes and genotypes suitable for linkage analysis were available for 5441 individuals (2110 males, 3331 females) from 2134 families. The sample comprised 3554 twins (including 1609 complete pairs, of which 95 were MZ), 1684 non-twin siblings, and 203 children of twins (including 1 DZ pair). The number of quasi-independent sibling pairs (QISPs) in the dataset was 5067. Lifetime abstainers (i.e. individuals who had never tried alcohol) were excluded from the study and are not included in these sample numbers. Genotyping was available for 1642 parents (including 599 parental pairs).
