Question

Below are possible alleles found in the lac operon.

Repressor gene:

I^s (superrepressor; can't bind allolactose, but can bind operator)

I⁺ (inducible; wild-type)

I^- (constitutive; can bind allolactose, but can't bind operator)

Operator:

O^c (constitutive; can't bind repressor)

O⁺ (inducible; wild-type)

A. Which of the following combinations below will not express β-galactosidase even when lactose is present? Circle and explain your answer.

I⁺O⁺

I^-O^c

I^sO^c

I^sO⁺

I^-O⁺

B. Which of the following combinations below will express β-galactosidase only if lactose is present? Circle and explain your answer.

I⁺O⁺

I^-O^c

I^sO^c

I^sO⁺

I^-O⁺

Note: This question is for an INTRODUCTORY genetics class, so please try to answer in a way that an INTRODUCTORY student would be able to understand.