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Mutational Genetics - A strain of Aspergillus was subjected to mutagenesis by X rays

A strain of Aspergillus was subjected to mutagenesis by X rays, and two tryptophan-requiring mutants ( A and B) were isolated. These tryptophan-requiring strains were plated in large 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 precisely reversed the original change that produced the typ- mutant allele?

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

Propose an explanation of why no revertants from mutant A were recovered.

Solution Preview

(a) If the revertant precisely reversed the original mutation that produced the typ- mutant, then clearly, the revertant is completely wild type in the original way, not just wt with respect to phenotype, but wt with respect to genotype as well. Get my point? In other words, if the revertant reversed the mutation exactly like it was before the mutation occurred, then it's totally normal in every way. Thus, if this revertant is mated with a wt ...