Essential Genetics 6th Edition by Daniel Hartl

Exercise 1

A mutant strain of E. coli fails to make b -galactosidase even when lactose is present. Genetic tests show that this mutant is in the lacI gene and is dominant. The strain was exposed to UV light to induce additional mutations, and mutant cells were selected that could grow on lactose. Two of these new mutants are characterized and found to be constitutive for ?? -galactosidase expression. Mutation A maps to the lac operator region and mutation B maps to the lacI gene.
(a) Would mutation A be dominant or recessive to the wildtype allele?
(b) Briefly explain why mutation A suppresses the original dominant, noninducible mutation.
(c) Would mutation B be dominant or recessive to the wildtype allele?
(d) Briefly explain why mutation B suppresses the original dominant, noninducible mutation.