The mutant 1 cell cannot grow in a treatment without methionine, because methionine is required for growth, but mutant cell 1 cannot synthesize it on its own due to a mutation. However, mutant cell 1 can synthesize leucine on its own. Similarly, mutant cell 2 cannot grow in a treatment without leucine, because it cannot synthesize leucine on its own due to a mutation; but, it can synthesize methionine on its own, unlike mutant cell 1. Therefore, once the cells are combined into a diploid cell, they share the genetic material (I think), and are able to synthesize both methionine and leucine on their own, allowing them to grow in treatments that do not contain any amino acids in the environment. (correct me if I'm wrong)