lacked something needed for this reaction. Specifically, they lacked a certain enzyme—a protein that acts as a biological catalyst, allowing chemical reactions to take place rapidly at body temperatures.

Since the disease is genetic in origin, Garrod speculated that genes consist of instructions for making enzymes, and perhaps other types of proteins as well. His insight was right, but it wasn't confirmed until a series of clever experiments gave definite proof nearly 40 years later.

In 1941, Stanford University geneticists George Beadle and Edward Tatum made the breakthrough that indisputably tied genes to enzymes. They did this through a series of tests with genetically mutated strains of bread mold. Each strain lacked the ability to produce one of the essential nutrients (amino acids or vitamins) that fungi normally need to grow. This lack, in turn, was due to the absence of a necessary enzyme.

By growing different strains of bread mold on different dishes with different combinations of nutrients, the scientists determined exactly which particular enzyme was lacking in each mutant strain. At the same time, they found that each genetic mutation was located at a specific site on the fungal chromosomes. A different site (or, in other words, a different gene) was associated with each enzyme. The geneticists concluded that one gene produces one enzyme.

So the answer to the question ''what do genes do?" is that genes are instructions for making various proteins. Who can believe that the difference between blue eyes and brown eyes, or for that matter between a sheep and your next-door neighbor, comes down to that?