Eight independently isolated mutants of E. coli, all of which are unable to grow in the absence of histidine (hisˆ’), were examined in all possible cis and trans heterozygotes (partial diploids). All of the cis heterozygotes were able to grow in the absence of histidine. The trans heterozygotes yielded two different responses: some of them grew in the absence of histidine; others did not. The experimental results, using €œ+€ to indicate growth and €œ0€ to indicate no growth, are given in the accompanying table. How many genes are defined by these eight mutations? Which mutant strains carry mutations in the same gene(s)?

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Growth of Trans Heterozygotes (without Histidine) Mutant 7