Final answer:
According to Beadle and Tatum's hypothesis, 3 genes are necessary for the arginine biosynthesis pathway.
Step-by-step explanation:
According to Beadle and Tatum's hypothesis, each mutant in the arginine biosynthesis pathway is defective in a different gene encoding a different enzyme. Therefore, based on their experiments, it can be determined that 3 genes are necessary for this pathway.