a. With nic-2 allele 1, there was no prototrophs obtained. From this result we are able to understand that some chromosomal rearrangements like either a deletion or an inversion would have occurred within the gene.
b. Each prototrophic colony was crossed separately with a wild-type strain. So, half of the progeny should come from the wild-type parent in each cross.
Prototroph A: The result of the cross was 100% prototrophic progeny. In this case a reversion at the original mutant site would have happened.
Prototroph B: The result of the cross was 50% of the progeny. They are the parental prototrophs, while the remaining 28% prototrophs are the result of the new mutation. It should be noticed that 28% of prototrophs and the 22% nictotinamide requiring autotrophs are approximately equal. We can infer from the results that an unlinked suppressor mutation would have occurred, resulting in independent assortment with the nic mutant.
Prototroph C: There were 996 prototrophs and 4 autotrophs. This result shows that a suppressor mutation would have happened very close to the site of original mutation. This suppressor mutation would not have been separated frequently from the original mutation by recombination.