Twin studies and other family studies such as the ones described above have suggested that genetic factors impact both alcohol dependence and smoking. To extend the family studies of these disorders and systematically identify genetic regions that contribute to alcohol dependence, habitual smoking, or both, COGA researchers used a method known as genetic linkage analysis. This approach involves searching for DNA sequence variations throughout the genome, using DNA sequences (i.e., markers) whose location on the chromosomes is known. Hundreds of markers have been identified in recent years that cover all human chromosomes. In many cases, several variants, or alleles, exist for a given marker, so researchers can determine not only whether the marker is present or absent in family members but also whether family members share a particular marker allele or not.
