The enzyme tryptophan synthase is composed of two polypeptide subunits each encoded by one gene in neurospora crassa. a mutant produces no tryptophan synthase due to a single point mutation in each of the genes (a and b) encoding the two protein subunits. a researcher is interested in identifying a revertant which produces fully functional tryptophan synthase. if the reversion rate for point mutations in n. crassa is 1 x 10-6 per haploid gamete, how many spores, on average, would the researcher need to screen to identify one revertant with the genotype a+ and b+ (functional alleles) which therefore can synthesize tryptophan synthase? a. 106 b. 2x106 c. 1012 d. 10-6 e.