Case–control studies compare allele frequencies in a sample of alcoholic and control subjects. Because DNA is inherited from both parents, every person carries two copies of the DNA at a given position in the genome—one allele that was inherited from the father and one allele that was inherited from the mother. The genotype describes the variation at a particular position within the genome and is defined by the allele inherited from the father and the allele inherited from the mother. If a given allele contributed to the risk for alcohol dependence, one would expect the allele and/or genotype frequencies to differ between the case and the control subjects (see figure 1A).
