Good Gene, Bad Gene: It Depends

Let’s leave aside for now the ethics of making designer babies—you know, blocking a “bad” gene or inserting a “good” one in a fertilized egg so you can pick a child's traits. Bad gene presumably include those for greater risk of cancer and other diseases, while good genes include those for traits like intelligence, good health and a winning personality. As a purely scientific proposition, the idea has a little drawback that keeps getting bigger. There is almost no such thing as a good gene or a bad gene, but only genes that are good or bad depending on the other genes around them and the environment you live in.

The classic example of this is sickle cell, the genetically-based blood disorder. It can be fatal, but in some settings it can be a life saver: if you have the sickle-cell gene and live in a malarial zone, you are less likely to contract malaria. Similarly, having one copy of the gene that causes cystic fibrosis seems to protect against tuberculosis. That’s why seemingly “bad” genes have survived in the human gene pool.

Now comes another reason to doubt the good-gene/bad-gene simplification. Minor variations in two genes that make receptors for the brain chemical dopamine—they’re called DRD2 and DRD4—have been linked to all sorts of undesirable things: attention deficit hyperactivity disorder, substance abuse and food craving, for instance.

But the same “bad” gene might be beneficial depending on how people live, finds a new study in the open access journal BMC Evolutionary Biology.

The variations in the DRD2 and the DRD4 genes affect how many dopamine receptors are made or how well they work in the brain. That, in turn, has been shown to affect behaviors such as

But when scientists looked at two groups of people—Ariaal tribesmen of northern Kenya, some of whom are nomads and some who are settled farmers and pastoralists—the effects of the genes turned out to be strikingly different. Nomads with the “bad” form of the DRD4 gene (associated with ADHD and food craving) turned out to be in generally better health and to be better nourished than those without the bad form, but farmers with the bad form suffered from malnutrition.

If they had studied the gene only in nomads, scientists would have called it beneficial. if they had studied it only in the farmers, it would be called detrimental. By studying it in both populations, the research shows that whether a gene is good or bad depends on the rest of a person's genes and the lifestyle he lives.

“Some of the variety of personalities we see in people is evolutionarily helpful or detrimental, depending on the context,” said Dan Eisenberg, an anthropology graduate student at Northwestern University who led the study. “This insight might allow us to begin to view ADHD as not just a disease but something with adaptive components.”

Why the difference? Eisenberg speculates that a nomadic man with the ADHD, food- and drug-craving form of the DRD4 gene “might be able to more effectively defend livestock against raiders or locate food and water sources, but that the same tendencies might not be as beneficial in settled pursuits such as focusing in school, farming or selling goods.”

Consider it another cautionary tale next time someone starts talking about tinkering with the human gene pool. The bad genes you think you’re eliminating just might be beneficial, while the good ones might be harmful, depending on genetic background and environment.