Genetics and addiction: scientists are uncovering new information from the field of genetics that will be able to help in the prevention and treatment of drug addiction.

• Position: HEADS UP REAL NEWS ABOUT DRUGS AND YOUR BODY

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John Crawford has an identical twin. His 16-year-old brother, Keith, is his mirror image. John has brown hair, brown eyes, and a slightly crooked smile. So does Keith. John weighs about 160 pounds. Ditto for Keith. The brothers walk the same, talk the same--their friends tease them because they even bowl the same.

The reason, of course, is that identical twins share exactly the same genes, those tiny units of hereditary material (DNA) that carry instructions for forming all the cells in the body and directing their activity. Beyond their physical similarities, the brothers also share less-obvious traits. These range from harmless things such as food preferences--they both love hot chilies--to unseen vulnerabilities, such as a risk for heart disease, diabetes, and drug addiction.

You've probably heard that drug and alcohol abuse runs in families. But what exactly does that mean? John and Keith have a parent who struggles with alcoholism, so are they destined by genetics to face the same fate? The good news is that no single factor determines whether a person will become addicted to drugs. That's because genetics, biology, and environment all influence a person's risk for drug addiction, defined as a chronic yet treatable brain disease characterized by compulsive drug seeking and use.

So, while the saying may be that substance abuse "runs in the family," a whole list of other risk factors in addition to genes come into play to determine whether a person gets hooked. These include biological factors, such as genetics, one's age, or the presence of other diseases, as well as environmental factors, such as diet, stress, or peer pressure. Neither John nor Keith shares their parent's battle with addiction, but the twins know that their family genetics can increase their risk of addiction.

It may be hard to see how a tiny piece of DNA could influence the risk of becoming addicted. The fact is, however, that scientists have identified several genes that can do just that. In reality, every gene comes in different forms, or variants, and researchers have discovered that some of these variants can make people more likely to abuse marijuana, heroin, cocaine, and other illegal, as well as legal, drugs. Certain genes can actually influence the way in which a person responds to drugs.

For instance, Harvard University scientists recently studied twins to gauge their responses to marijuana. They studied 352 pairs of identical twins...