Studies of inferred genetic risk described in the previous section examined how the environment can potentiate or limit variation in drinking behaviors for the aggregate effects of genotypes. However, these studies offer no knowledge about which genes contribute to these interactions and were not designed to do so. In this next section, we review studies examining interactions between specific genetic loci and measured environmental factors. First, we provide a definition of a single nucleotide polymorphism (SNP) for clarification. A SNP is a DNA sequence variation occurring when a single nucleotide (A, T, C, or G) differs between paired chromosomes in an individual. For example, for a CT SNP, we say that there are two alleles: C and T and that this individual has the CT genotype. Other possible genotypes for individuals in a population are CC and TT. Readers interested in more detailed information are referred to Carey (2002) and the International HapMap Project website.
