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What Exactly Do Genes Do?

Image of DNA.

By the 1940s biologists assumed that information of some kind is transmitted from generation to generation by the so-called genes, but they still didn’t know precisely what genes did. (Wikimedia Commons)

The classic experiments in the history of science are often memorable not for complexity but for cleverness. Here’s an example.

By the 1940s biologists were pretty well convinced that offspring grow to resemble their parents because information of some kind is transmitted from generation to generation by the so-called genes. But what information is transmitted? Exactly what does a gene do?

The strategy of the biologist George Beadle and Edward Tatum was to “find out what genes do by making them defective.” Beadle and Tatum worked in the 1940s with ordinary bead mold.

They grew mold in test tubes with a simple nutrient formula: sugar, minerals and one vitamin.

Normal bread mold could make all the proteins and other substances it needed from that nutrient formula.

Then Beadle and Tatum exposed some of their mold samples to intense X-rays.

A few of the X-rayed molds would no longer grow unless they had proteins added to their diet. The X-rays had destroyed the molds’ ability to make those proteins.

What’s more, this inability to make certain proteins was passed on to future generations of molds like any other inherited trait. The X-rays had caused a defect, and the defect was inherited. The real effect of the X-rays had been to damage the molds’ genes.

The Beadle-Tatum experiment was important because it showed that what a “gene” conveys from generation to generation is a set of recipes or instructions for making proteins.


  • Beadle, “The Genes of Men and Molds,” Scientific American, September 1948; reprinted in Genetics: Readings from Scientific American, intro. by C. Davern (1981).

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