Final answer:
The number of genes necessary for a simple metabolic pathway, according to Beadle and Tatum's hypothesis, would be equal to the number of steps in the pathway; for a pathway with three steps, this would mean three genes are necessary.
Step-by-step explanation:
According to Beadle and Tatum's hypothesis, which is often summarized as the one gene-one enzyme hypothesis, each gene is responsible for producing a single enzyme that in turn affects a single step in a metabolic pathway. In the provided figure of a simple metabolic pathway, this would suggest that each step in the pathway is catalyzed by a different enzyme encoded by a different gene. Therefore, if the pathway illustrated has three steps, then according to this hypothesis, three genes are necessary for the pathway.