To figure out which metabolic pathway was "broken" in each mutant, Beadle and Tatum performed 3 Steps

First, they grew each mutant on minimal medium supplemented with either the full set of amino acids or the full set of vitamins (or sugars, though we won't examine that case here). If a mutant grew on minimal medium with amino acids (but not vitamins), it must be unable to make one or more amino acids. If a mutant grew on the vitamin medium but not the amino acid medium, it must be unable to make one or more vitamins. If a mutant grew on minimal medium containing all 20 amino acids, they might next test it in 20different vials, each containing minimal medium plus just one of the 20 amino acids. If the mutant grew in one of these vials, Beadle and Tatum knew that the amino acid in that vial must be the end product of the pathway disrupted in the mutant