After the diagnostic assessment, a subset of families with at least two alcoholic members in addition to the initially recruited proband were invited to participate in a second stage of assessment, which included blood collection for genetic analyses. For these analyses, the investigators checked a total of 336 short, repeated DNA sequences located throughout all chromosomes (for more information, see Reich et al. 1998; Nurnberger et al. 2001). These sequences are useful as markers because they vary in size from one person to another and their inheritance pattern can therefore be easily determined. This screening process was carried out with two groups of participants. The first group (the “initial data set”) included 987 people from 105 families, and the second group (the “replication data set”) included 1,295 people from 157 families.
