No one is born an addict.
When scientists look for "addiction genes," what they are really looking for are biological differences that may make someone more or less vulnerable to addiction.
It may be harder for people with certain genes to quit once they start. Or they may experience more severe withdrawal symptoms if they try to quit. Factors that make it harder to become addicted also may be genetic. For example, an individual may feel sick from a drug that makes other people feel good.
But someone's genetic makeup will never doom them to inevitably become an addict. Remember, environment makes up a large part of addiction risk.
Scientists will never find just one single addiction gene. Susceptibility to addiction is the result of many
Social and environmental factors contribute to this risk of addiction. It is becoming increasingly clear that genetic factors also weigh in.
Like other behavioral diseases, addiction vulnerability is a very complex trait. Many factors determine the likelihood that someone will become an addict.
"Just because you are prone to addiction doesn't mean you're going to become addicted. It just means you've got to be careful."
Dr. Glen Hanson
Try your hand at building a pedigree for a family afflicted with nicotine addiction. Watch video interviews, and interpret surveys to discover if smoking could be "in the genes" for the Marshall family.
There are many ways that genes could cause one person to be more vulnerable to addiction than another.
Dr. Glen Hanson
Researchers construct pedigrees of large families with addiction as a first step to understanding the disease.
A pedigree can reveal whether or not a trait has a genetic component. That is, whether or not it is passed down from parent to child by way of genes.
Because addiction is influenced by multiple genes as well as environmental risk factors, this can be a complicated process.
Various genes and environmental factors can add up or cancel each other out. Not every addict will carry the same gene, and not everyone who carries an addiction gene will exhibit the trait.
Below you will find a sampling of some of the genes likely to play a role in addiction.
The A1 allele of the dopamine receptor gene DRD2 is more common in people addicted to alcohol or cocaine.
Increased expression of the Mpdz gene results in mice experiencing less severe withdrawal symptoms from sedative-hypnotic drugs such as barbiturates.
Mice bred to lack the cannabinoid receptor gene Cnr1 have a reduced reward response to morphine.
Mice bred to lack the serotonin receptor gene Htr1b are more attracted to cocaine and alcohol.
Mice bred to lack the β2 subunit of nicotinic cholinergic receptors have a reduced reward response to cocaine.
Mice with low levels of neuropeptide Y drink more alcohol, whereas those with higher levels tend to abstain.
Fruit flies mutated to be unable to synthesize tyramine remain sedate even after repeated doses of cocaine.
Mice mutated with a defective Per2 gene drink three times more alcohol than normal.
Non-smokers are more likely than smokers to carry a protective gene, CYP2A6, which causes them to feel more nausea and dizziness from smoking.
Alcoholism is rare in people with two copies of the ALDH*2 gene variation.
Mice with the Creb gene "knocked-out" are less likely to develop morphine dependence.
Using pedigree data, researchers can begin to hunt down genes. They begin by comparing DNA sequences of individuals who have the disease with those who do not. They can then narrow down the possibilities to identify a small number of so-called "candidate genes" for addiction.
Most of the genes at right wouldn't have been discovered without the use of animal models, especially mice.
Because the reward pathway functions in much the same way in mice as it does in people, mice are leading the way in identifying addiction genes. When researchers discover a gene in mice that plays a role in addiction, they can then identify the counterpart gene in humans by comparing DNA sequences.
Developing improved treatments for addiction is now becoming easier thanks to the discovery of addiction susceptibility genes. Each new addiction gene identified becomes a potential "drug target." That is, researchers can focus on one gene product and develop a drug that modifies its activity. In so doing, signals or pathways in the brain may be reversed or stabilized to restore proper brain function.
The effectiveness of addiction medications vary from person to person due to differences in genetic make-up. In the future, genetic tests could be used to determine which genetic variation a patient has and prescribe the best addiction medication for that individual. Learn more in Pharmacogenomics: Drugs Designed for You.