A strain of Aspergillus was subjected to the mutagenesis by X rays, and two tryptophan-requiring mutants (A and B) were isolated. These tryptophan-requiring strains were plated in huge numbers to obtain revertants to wild type. You failed to recover any revertants from mutants A and recovered one revertant from mutant B. This revertant was crossed with a normal wild-type strain. What proportion of the progeny from this cross would be wild type if:

a) The reversion accurately reversed the original change that produced the typ- mutant allele?

b) The revertant phenotype was generated by a mutation in a second gene located on a dissimilar chromosome (the new mutation suppresses trp-)?

Suggest a description of why no revertants from mutant A were recovered.