Most family, twin, and adoption studies have shown that addiction to alcohol has significant heritability. For example, there is an increased risk for alcoholism in the relatives of alcoholics. Depending on the study, the risk of alcoholism in siblings of alcoholics is between 1.5 and 4 times the risk for the general population. The identical twins of alcoholics (who share 100
percent of their genes) are more likely to be alcoholics than the fraternal twins of alcoholics (who share only about 50 percent). Adoption study data suggest that the risk for developing alcoholism for adopted children is influenced more by whether their biological parents were alcoholics than whether their adopted parents are alcoholics, suggesting that genes contribute to alcoholism more than environment. Similar but less extensive data has been collected for nicotine addiction. Very little genetic epidemiological data has been collected for illegal drugs.
The only genes that have been conclusively shown to affect susceptibility to addiction in humans are genes that encode proteins responsible for the metabolism of alcohol. In the body, ethanol ("drinking" alchohol) is oxidized by enzymes to acetaldehyde and then to acetate. Certain alleles of aldehyde dehydrogenase genes that are common in some populations, such as Asians, lead to increased levels of acetaldehyde when alcohol is consumed. Acetaldehyde causes an unpleasant flushing reaction that leads to a voluntary reduction of alcohol consumption. The systematic search for other genes that affect susceptibility to alcohol and nicotine addiction in humans has lead to the identification of chromosome loci that may contain genes that affect susceptibility to addiction, but has not lead to the identification of any specific genes.
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