Both alcohol phenotypes and genotypes suitable for linkage analysis were available for 4172 individuals (1654 males, 2518 females) from 1690 families. The sample comprised 3085 twins (including 1465 complete pairs, of which 34 were MZ) and 1087 non-twin siblings. The number of quasi-independent sibling pairs (QISPs – i.e. twin pairs plus pairings formed between twins and their non-twin siblings) in the dataset was 3837. Genotyping was available for 1338 parents (including 483 parental pairs) and these were used to refine estimates of IBD probabilities.
