Genetics and addiction, can the propensity for addiction be inherited? This is a question that scientists have been puzzling over for years.
Most researchers have observed that there may be a link, though not a definitive one, between addiction and genetics. For example, people whose brothers, sisters and parents were alcoholic are more likely to be alcoholic than people whose first degree relatives didn’t have this problem. However, in that case the person could have been influenced because he or she grew up in a household where alcoholism was modeled. Of course, the alcoholism might have resulted from the interaction of both genes and environment.
No One Gene
Right now, there’s no one gene for alcoholism or any other sort of addiction. However, scientists can investigate a person’s liability to developing addiction through both behavioral genetics and molecular genetic research. In behavioral genetics, scientists may look at the behavior of families, twins and adoptees. This can give a scientist a rough estimate as to how genes and environment interact to lead a person to addiction. Molecular genetic research looks at the person’s genome to measure variations in liability to addiction and to see if any particular genes have a role to play in addiction. A person’s genome is his or her complete set of genes.
Researchers found that heritability of liability for addiction is quite high among both men and women. Heritability is the interaction of both genes and environment when it comes to putting someone at risk for addiction. Studies have found that the biological parents of adopted children who are addicts were very likely to have a substance abuse disorder themselves. This is true even if the children were separated from their parents shortly after birth. But there was no relationship between the substance the parent was addicted to and what substance the child became addicted to. Twin studies also showed that if one twin was addicted to a substance, the other twin had a 60 to 70 percent chance of being addicted as well.
Twin studies are important because identical twins have exactly the same genes. This is because they were created when a single zygote divided in two. Any major differences between them are the result of their environment. On the other hand, fraternal twins are just like any sibling pair and only share 50 percent of their genes. If addiction is really strongly controlled by genes, then identical twins will either use or not abuse drugs whether they were raised in the same environment or not. With fraternal twins, one twin might not use drugs while the other is addicted.
Studies of the children of identical and fraternal twins show that if their parents abuse drugs, they’re also at high risk for abusing drugs because of their environment and because they’ve inherited genes that have put them at greater risk for addiction. Children of an identical twin who doesn’t abuse drugs have a lower environmental risk for addiction, but a high genetic risk. Children of non-using fraternal twins have a lower environmental risk and an indeterminate genetic risk.
These studies are important to know because they can help scientists pinpoint both the genes that can contribute to liability as well as environmental factors.
Studies on Animals
Scientists are able to manipulate both the genes and environment of lab animals. Still, the results are still uncertain. Some lab mice were created with an inactivated dopamine receptor gene. Dopamine is a neurotransmitter that’s tied in to the brain’s reward system. The reward centers of the brains of these mice didn’t react when they were given morphine, but did react when they were given food. Other mice with another inactivated dopamine receptor reacted to substances like cocaine and alcohol. The implications of this for humans are yet to be seen.