This is very similar to Epistasis, except that the scenario here is different.
This is the experiment that proved one gene = enzyme
The main idea is that the organism has to be able to convert the media they're on into arginine in order to see growth or survive. So if adding a media doesn't show growth, that means an enzyme downstream isn't working to the point that the organism cannot make arginine to survive.
At the end, if you're confused as to why does the genes from Arg 1 not rescuing or saving the mutations from Arg 2. What you're actually seeing is a new mutant with 2 of the mutations from each Arg mutant. The Arg 1 and 2 mutant would instead have a genotype that looks like this:
E-/E- ; F-/F- ; G+/G+ .
Same way for Arg 1 and Arg 3 mutant:
E-/E- ; F+/F+ ; G-/G-
Basically, the combinations at part 5 is just literally making a new mutant with genotypes containing the mutations from the previous isolated arg mutants.
