The following table shows several bacterial strain lac operon genotypes (some are partial diploids).
a. Fill in the blanks in the "lactose absent" and "lactose present" columns in this table. (+) means significant levels of active ß-galactosidase enzyme can be detected. (-) means no significant levels of active ß-galactosidase enzyme are present. The first line is filled in for reference.

\begin{array}{|l|c|c|}\hline\text { Strain genotype }&\text { Lactose absent } & \text { Lactose present } \\\hline 1. l a c I^{+} l a c P^{+} l a c O^{+} l a c Z^{+} l a c Y^{+} &-&+\\\hline \text { 2. } \operatorname{lacI}^{+} \operatorname{lac}^{+} \operatorname{lac} O^{+} \operatorname{lacZ^{+}} \operatorname{lac} Y^{+}\\\hline 3. l a c I^{+} l a c P^{+} \operatorname{lac} O^{+} \operatorname{lacZ^{-}lacY^{+}} \\4. l a c \mathrm{l}^{*} l a c P^{+} \operatorname{lac} O^{+} \operatorname{lacZ}^{+} l a c \mathrm{Y}^{+} \\\text { 5. } \operatorname{lacI}^{-} \operatorname{lacP^{+}} \operatorname{lacO^{+}\operatorname {lac}Z^{+}lacY^{+}/lacI^{+}}\\\text { 6. } \operatorname{lac} I^{\top} \operatorname{lacP^{+}} \operatorname{lac} O^{+} \operatorname{lacZ^{+}} \operatorname{lac} Y^{+} / \operatorname{lac} I^{+}\\\hline\end{array}

b. Using your answers in the previous table, explain the dominance relationships among the three lacI alleles (lacI⁺, lacI⁻, lacI^s).

An Operon Is Controlled by a Repressor

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