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Tuesday, May 17, 2005

Can a gene really make you fat?

Many people have worried a lot about genetic determinism—the belief that there are master genes that control our behaviour — crime, obesity, sexuality, honesty ...

There are two separate worries here: First, is it true? If so, free will does not really exist.

More practically, what if large numbers of people believe it is true, even if it isn’t? Then those people will act as if they, and we, have no free will. They will have a bad effect on society even if they are wrong.

Fortunately, recent developments in genetics are making clear that this sort of “reductionism” — reducing everything to a single factor — is nonsense. There are no single genes that control human behaviour. Popular literature often does not catch up with new science developments very quickly, so it might be helpful to summarize a couple of key points here:

Not very long ago, as MIT science historian Evelyn Fox Keller notes in The Century of the Gene, biologists thought that if we could read the human genome, it would explain everything there was to know about a person. As Francis Crick put in in 1957, “DNA makes RNA, RNA makes protein, and proteins make us.” This was the “central dogma” of biology for nearly fifty years.

Quoting another scientist from about 15 years ago, Keller says,

Spelling out his ‘Vision of the Grail,’ Walter Gilbert wrote, “Three billion bases of sequence can be put on a single compact disc (CD), and one will be able to pull a CD out of one’s pocket and say, ‘Here is a human being; it’s me!’"


Then she adds, “Today, almost no one would make such a provocative claim.”

So why would almost no one make such a claim today? Both genes and organisms have turned out to be much more complex than anyone imagined. As Keller notes, “Indeed, the functional gene may have no fixity at all: its existence is both transitory and contingent, depending critically on the functional dynamics of the entire organism.”

As a recent article in the Guardian Education supplement explained, most genes do not do only one job:

Rather than having a single major function, most genes, like roads, probably play a small part in lots of tasks within the cell. By dissecting biology into its genetic atoms, reductionism failed to account for these multitasking genes. So the starting point for systems biologists isn't the gene but rather a mathematical model of the entire cell. Instead of focusing on key control points, systems biologists look at the system properties of the entire network. In this new vision of biology, genes aren't discrete nuggets of genetic information but more diffuse entities whose functional reality may be spread across hundreds of interacting DNA segments.


Put simply, if you have a tendency toward inactivity that causes you to gain weight, there isn’t a “gene” that makes you fat. Your tendency is likely the result of a system with hundreds of components, interacting with other systems with hundreds of components. The bad news is that it is more difficult to understand than a simple system. The good news is that it is much easier to influence than a simple system, because you can influence many different components. So biology is not destiny after all.

Of course, we can expect years of headlines about the “shoplifting gene,” the “compulsive spending” gene and so forth. Promising ideas die hard. That little word “gene” promises us absolution without ever having to say we’re sorry—or even admit that we did something bad.

To find out more about my book about the intelligent design controversy, go to By Design or by Chance?

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