In addition to pursuing the candidate gene approach when searching for susceptibility genes for alcohol dependence, researchers have undertaken a more comprehensive search of the entire genome. In the cell, the DNA is packaged into segments, or chromosomes. Each human cell contains 23 pairs of chromosomes,5 one set inherited from the mother and one from the father. To screen the entire genome for DNA sequences related to a disease, scientists first identify numerous DNA sequences (which may or may not contain genes) whose locations on the chromosomes are known and which can serve as genetic markers. The goal is to identify enough markers to cover several regions on each chromosome. Subsequently, the researchers test how commonly these markers are shared by siblings who are both affected with alcohol dependence or habitual smoking. If a certain marker is shared more often by affected siblings than can be expected by chance alone, the marker is said to be “linked” with the disease.
